Junk filtering has become one of the most important features of email frontends.
Webmailers like GMail do this automatically: If a user rates a message junk, it will be rated as possibly junk for all users. The more active users, the better the junk filter. If I am the only one who classifies my email (like with Thunderbird) I will never get a good junk filtering.
Thunderbird has no central instance (as far as I know of). A common Junk classification would need a cloud backend. I don't have a problem with that. I am even happy to pay for a service like that.
I might not be the first with this idea. If this is a duplicate, I apologize. Otherwise I am happy for upvotes 😉