Thankfully we have %:z. Relatedly, it’s possible to teach date to do this by default: https://gist.github.com/d081dad407432d53172e30d0d35c39db

  $ date
  2023-08-31T11:15:00-07:00

+%:z is a gnu-specific extension, though. The gist you linked works with gnu libc, but I'm not aware of a POSIX-compliant way to do it.

> timezone is specified as 4 digits without the colon

±NNNN is valid (without colon) if used as part of the "basic" format. i.e. no hyphens or colons anywhere in the format.

Thus the following are equivalent and both valid:

    2023-09-01T09:40:01+08:00
    20230901T094001+0800

Ah, I see that listed as a valid ISO 8601 format in the table, but not in the visualization, so I missed it. Also I can't say I've ever seen the basic format in the wild, is there anywhere it's used regularly?

When the timestamp gets slammed into a file name, as the colon is reserved on Windows and MacOS. Also, when space matters, as it's very slightly shorter.