I guess? This is an anonymity feature no other mainstream messenger has in the first place. The idea behind sealed sender is that clients can send messages through Signal's service without authenticating directly to the server. It's not, like, part of the core E2E mechanic of Signal; I'm not even sure it was ever out of beta.

The suggested fix was pretty complicated and had some drawbacks (see section VII-B). Signal probably did not agree that it was worth it. It's a hard problem and it just might be the case that no one has of yet come up with a workable solution.