For starters, it was never "open source"...

From your link:

Instead, as Winamp CEO Alexandre Saboundjian said, "Winamp will remain the owner of the software and will decide on the innovations made in the official version." The sort-of open-source version is going by the name FreeLLama.

While Winamp hasn't said yet what license it will use for this forthcoming version, it cannot be open source with that level of corporate control.

If I upload the source code for my project on Github/Forgejo/Gitlab/Gitea and license it under and open source license, allowing you to fork it and do whatever you want (so long as you follow the terms of my copyleft license), and I diligently ensure that code is uploaded to my repository before being deployed, but I ignore all issues, feature requests, PRs, etc., is my project open source?

Yes.

Likewise, if Winamp had been licensed under an open source license, it would have been open source, regardless of how much control they kept over the official distribution.

Winamp wasn’t open source because its license, the WCL, wasn’t open source.