Sure it is the decent thing to do to give them a reason, but also invites arguments so I'm of two minds.  They will almost inevitably disagree with your reason and get very angry.  Relay operators are humans and can't deal with angry people day after day, and being kicked off of a relay in nostr shouldn't be a big deal, you can just trivially go somewhere else.  If I was booted from a relay I'd get very excited and happy because "fuck you bitch I'm using nostr, and you can't stop me!"  and it would feel great to have such a retort.... It is like sometimes I wish a nuclear war would actually happen so I could feel good that I moved to New Zealand.... if nobody ever tries to censor me, what was the point of nostr?    So being explained to isn't really something I think relay ops need to do.  Maybe they just want to save on ISP costs (traffic, disk space).

Giving them a chance to download their data is also the decent thing to do, but probably not necessary in practical terms because ... well, because you are using lots of relays aren't you?

I recommend against posting to 17 or 25 relays only because it does not seem like the decent thing to do.  I don't plaster all the walls of your home with posters of my missing cat for the same reason, I just put it on a few telephone poles. 

 I would be interested to know how much it costs a relay operator to host my individual data, to get a sense of how much I should contribute to them, at a minimum. 🤔 If it cost a lot, that could get me to cut back on my number of relays! 😄

I've actually thought about creating a new npub and adding tons and tons of relays, as an experiment, to see if it would enhance or degrades my Nostr experience... 🧑‍🔬 

 As of now people are fairly generous and the cost of being on a public relay is probably not very much.  But if every event is sent to every relay, scaling problems arise.

I think of this situation as analogous to posting a blog onto a blogging website, or onto my own website.  I can follow 100 different blogs on 100 different personal blog websites.  Or maybe I can follow them on a few centralized blogging websites like medium, substack, etc.  And it works suprisingly well with zero redundancy (just one website).   So if it works so damn well with zero redundancy, do we really need to be 25x redundant?  Even being just 3x redundant is a huge improvement over a blog on a website.  Being 5x redundant is really a hell of a lot safer.  I can't imagine a scenario where anyone would need more than 7x redundancy (except highly hated people like Alex Jones and Donald Trump).

Except actually I can imagine such a scenario.  If it came to be that popular nostr clients didn't use the outbox model.  Then people would have to post to all the popular relays too.  But even then, I don't see the benefit of going beyond 7x and just picking some of those to be the popular relays, and instead I start to see it as maybe being disrespectful, taking advantage of the goodwill of open relay hosts because you can...  potentially leading to those relays to shut down due to being overloaded.

And many have predicted all relays will eventually charge for service. lest they be taken down for illegal content or be flooded with excessive traffic that the operator cannot afford on their goodwill. It might come to pass. 

 'I can't imagine a scenario where anyone would need more than 7x 
redundancy (except highly hated people like Alex Jones and Donald 
Trump).'

Well given that Trump  is running for president (no matter how much anyone hates him) I think this makes for a very strong argument against Nostr being hugly censorship resistant.  It would be easy for 7 relays to all agree on using the same censorship lists.   In fact I would say the ability for Donald Trump to never get censored on Nostr on most peoples clients would be the bare minimal needed to claim Nostr is actually censorship resistant . 

 The biggest problem with paid for relays for non bitcoiners isnt the cost so much as  having to disclose yourself for payment details (reduced anonymity) , inconvenience (its boring and complicated buying bitcoin).  And cost is still a factor for some when there are free alternatives , but for everyone when they aren't sure if they will even like Nostr.  Subscriptions are a faf.  There are so many,  getting increasingly frustrating and difficult to manage. It doesn't feel like you won your data when you have to pay.  That doesn't make Nostr sound as appealing as it once was.  One of the big fears against paying for Twitter is that it removed anonymity.  I suspect a lot of people don't pay more because of that than cost.  How do we verify relays aren't spying on us and linking our payment details to our posts?   I think being asked to pay for nostr will kill it.  Its a major obstacle for so many reasons, but even when it is free right now people are reluctant to switch to nostr. 

 If its trivial to go somewhere else after being  kicked of a relay, then why would any relay operator ever bother kicking anyone off for any reason.    Either kicking someone off solves a problem or it doesn't.   Either they can post or they cannot.  It makes zero sense that a relay operator would waste time banning anyone or anything if its trivial to carry on posting on a different relay. 

 I feel like this could become a bit like the 51% bitcoin problem only much easier:  if we have even as many as 100 relay operators used by most people it wouldn't take much for all of them to agree to ban someone by sharing the same ban list  - which already happens with spam lists - even if not intentionally its likely that censored people will just get added to a 'censor list' without much thought. 

 You really don't need to, if clients implement the outbox model correctly, and unlike Bitcoin, where every node needs to keep a copy of every single transaction in order to maintain censorship resistance, consensus, and full auditability of the supply, that is not the case for Nostr. We ONLY have the censorship resistance piece to worry about, and not every relay can store every user's notes, so being selective about the relays you post to is both wise and courteous.

One of the best things about Nostr is that you can personally host your own content privately, too. This way, even if all 5(or however many you use) of the relays you currently post to decided to ban you, you can simply change your public relays to somewhere else and rebroadcast your notes from your private relay. The reality is, even this is very unlikely to happen unless you really did something deserving of the ban such that all 5 relay operators banned you in quick succession. 

 "even this is very unlikely to happen unless..."

nostr:nevent1qqsfe2uafrnpvk8kax8txylc54ees4z6cjecfucplhazf3pu7l7lwvgppemhxue69uhkummn9ekx7mp0qyghwumn8ghj7mn0wd68ytnhd9hx2tcpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhscd5ls7 

 The president of the US was banned on at LEAST 5 differnt social media platforms.  Thats the litmus test for censorship: Can Donald Trump post there. 

 Trump can setup his own relay.  Then he can choose to not ban himself.  He could probably easily afford a large network of relays worldwide.

Unlike mastodump, his relay network can't be "cut off" because clients contact relays directly. 

 People need to know where to look, they also need to know he was censored by the relays in the first place.  But he can't relay that message if hes censored.  Will folk have to scour the internet to check every time he doesn't tweet for a few days, or can the outbox model solve this automatically? 

 NIP-65 says "Clients SHOULD spread an author's kind:10002 event to as many relays as viable"  the idea being that there is no centralized index, but these events are small and one-per-person (replaceable) so they can go just about everywhere without much overhead.  That way people can be found as they move around.