Since it seems like it could be useful to have this information somewhere, here are the remaining failing GNU tests for join:

• 3c: no support for joins with a null field separator, -t "\0" (join: add support for -t '\0' #2881)
• 10b: broken comparisons with empty fields (join: fix default check order behaviour #2908)
• sp-vs-blank: \f (form feed) seems to be interpreted as a newline (join: operate on bytes instead of Strings #2851)
• 8-bit-t: 8-bit characters aren't valid unicode, cause a panic (first half: join: operate on bytes instead of Strings #2851, second half: join: add support for non-unicode field separators #2902)
• bigfield1 and bigfield2: very large integers cause parsing as a field number to fail ( join: "support" field numbers larger than usize::MAX #2882)
• invalid-j: test expects the invalid -j error to trigger before the missing args error (patched upstream, update GNU coreutils version in GnuTests workflow #2919)
• chkodr-4 and chkodr-7: GNU has relaxed order checks by default; see info join -n "General options in join" (join: fix default check order behaviour #2908)
• chkodr-5, chkodr-5b, chkodr-5c, and header3: slight differences in "not sorted" error message (join: print unsorted line in error message #2899)
• chkodr-5d and chkodr-5e: missing additional error messages (join: fix default check order behaviour #2908)
• z1, z4, z5, z6: -z flag (null-terminated lines) isn't supported (join: add support for -z option #2880)