The main thing is that some clients don't support long form notes.

I like what @Fabian does with Nostur: a tab for long form notes, so you can go there and see what's new from the people you follow. 

That is a good user experience because it happens inherently as part of the established habit of reading notes in your everyday client.  

 The entire point is that you are supposed to use different clients for different use cases. 

 I see using different clients as a user preference and client choice.

In any case, creating a kind 1 sidekick event would let users of “kind 1 only" clients know that they can go to their long form client to check the new content 

 Yes— in my own way— this rings true