Per https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT section 4.1.1 —
"ZIP files MAY be identified by the standard .ZIP file extension although use of a file extension is not required. Use of the extension .ZIPX is also recognized and MAY be used for ZIP files."
PeaZip (mostly) supports this variation and also supplies some background info: https://peazip.github.io/zipx-file-format.html
"WinZip's ZIPX standard is based on PKZip's ZIP format specifications, with only difference of employing newer data compression algorithms - BZip2, LZMA / LZMA2 (XZ), PPMd and others (superseding classic Deflate / Deflate64 algorithm) - and employing custom pre-compression filters (WavPack, MP3 and JPEG filter) to improve compression of images and other media data structures."
I generated the included test file in WinZip Pro 26 instead of just copying test.zip to test.zipx.