Followed (kind 3) and Muted (kind 10000?) work just like custom lists, except you cannot rename them or delete them. You can add as many custom lists as you want.  You can add new npubs to them.  You can also go to a person page and flip switches to add/remove the person from any of your lists.  And you can mark people as "private" on a list so when published it hides the fact they are on the list by putting them in the encrypted contents.

I will do a short video on it I think. 

 What happens if someone has a pubkey in a categorized list kind 30000 but it doesn't appear in his/her contact list kind 3?
Which clients support these new kinds besides Gossip? 

 You can do that. Gossip manages to get relays and find events for the intersection of all your lists, not just your following list. 

 Does that mean I can view the feed of each of my custom lists and not the feed of my main contact list? 

 Yes. I have a 'news' list that only has news sources. None of that is 'followed' and doesn't show up in my following contacts list.