Vim isn't ad-supported. Vim has one messsage that you might consider an ad, and the money doesn't support vim's development, it goes to charity. I think it also used to be possible to donate to vim directly to support the development, but I think that money goes to charity now too.
(I personally don't think "help poor children in Uganda by donating" is an ad).
I think in earlier vim versions, you could see a message if you opened vim without a file (vim -u NONE # to prevent .vimrc from doing whatever shenanigans you've set up)
Huh. My .vimrc is pretty simple and definitely doesn't include a "don't show donation messages" line, but I do see the message on the front screen when I do `vim -u NONE`. It's not there when I execute `vim`.
When you start Vim (without arguments) it says "VIM - Vi IMproved" .... "Help poor children in Uganda! type :help iccf<Enter> for information". I just noticed that it actually cycles a few through messages now (version 8.something), so you may have to restart a couple of times to see the message. But IIRC it used to always just display the "help children in Uganda" message.
Strangely, it doesn't do that on any machine I have access to, despite the fact that I can find nothing in my .vimrc file that would suppress it. If I run `vim -u NONe` as the sibling comment suggested then I do see the message.
(I personally don't think "help poor children in Uganda by donating" is an ad).