No, it literally is part of the ZIP standard [0]. I updated my original comment.
ZIPX archives are simply ZIP archives that have been branded with the X for the benefit of the users who may be trying to open them with something ancient, that doesn't yet support LZMA or the other new features.
When a 'hacker' (programmer) says 'zip file' they refer to the widely compatible implementation which is likely to work in any common implementation of archive handling software.
According to the above version table, that would conform to PKWARE 2.0, which as with the ODF file specification, limits packing methods to STORE and DEFLATE only; as mentioned in the standardization section and ISO/IEC 21320-1.
When you say 'zip' you should be saying 'zipx' at the very least, but you may as well be comparing a tar file to a tar.zstd file in the same case.
> may as well be comparing a tar file to a tar.zstd file in the same case.
It's not exactly hard to get a new compressor on a *nix system. It's either in the repos, or not so hard to compile. On non-nixes, there's usually a binary. Tarballs have not been limited to only tar.gz for a very long time, though a lot of people do choose to be conservative in how their distributed files are compressed.
> When a 'hacker' (programmer) says 'zip file' they refer to the widely compatible implementation which is likely to work in any common implementation of archive handling software.
You don't speak for every hacker, certainly not for me. If you're implementing ZIP these days and it's not, at the very least, capable of reading ZIP64 archives (forget about newer compression methods), then you're just creating obsolete software.
ZIPX archives are simply ZIP archives that have been branded with the X for the benefit of the users who may be trying to open them with something ancient, that doesn't yet support LZMA or the other new features.
[0] https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT