There's a difference between source available and open source. For example, actually being allowed to distribute modified versions is pretty damn important:
Restrictions
No Distribution of Modified Versions: You may not distribute modified versions of the software, whether in source or binary form.
No Forking: You may not create, maintain, or distribute a forked version of the software.
Official Distribution: Only the maintainers of the official repository are allowed to distribute the software and its modifications.
The official open-source definition expects more freedoms that just being able to see the source: the whole point of having the source isn't transparency, it's freedom. Freedom to fork and modify. Freedom to adapt the code to fix it and make it work for your use case, and share those modifications.
This doesn't let you modify the code or share your modifications at all.
People often use the OSI's Open Source Definition when using the term "open source". One of its criteria says "The license must allow modifications and derived works" which this license does not allow.
Everyone has a different opinion on what that means, some people get really angry when you don't use their (or some other group's) explicit definition of the term "open source" that nobody actually owns. If they want it to mean something really specific, they should use a registered trade name with a defined meaning. But that usually implies some kind of capitalism at work, which most FOSS zealots are very much against.
I guess how new are you talking? I think this said it was based on the 2019 release, but I haven’t heard much about recent releases. Winamp 2 was the classic one most people remember. Winamp 3 was a rewrite that was supposed to be better under the hood but a lot of people didn’t like it, mainly for the new interface it seemed. They jumped to Winamp 5 (2+3) to restore much of the old interface while keeping the capabilities of 3. I never had issues with 5 and continued to use it through Windows 7. Haven’t used Windows much since then so I don’t know how it runs now. There have been very rare point updates since AOL took over and later sold it, mostly bugfixes.