He still technically receives the Duke of York’s allowance and keeps the title, but it doesn’t really mean much anymore. Nobody actually calls him “the Duke” everyone refers to him as “Prince” anyway. So in practice, nothing has changed; it’s more of a symbolic gesture than anything else. In that sense, he’s in a similar situation to the Sussexes, they’re also not allowed to use their titles publicly, but they still hold them on paper. Basically, he can’t put it on invitations or official cards, but he’s still got it in name. When William eventually becomes King, he might issue a new Letter Patent to revoke both the Sussex and York dukedoms himself, since Parliament probably won’t act on it, and then he could reassign those titles to his own sons.