>AFAIK the general practice is to use ! for methods that perform a mutation in-place The ! suffix is rather loosely defined as it just means the user should be cautious/the method might have "dangerous"/surprising behavior.
Your description is usually correct but only when a non-suffixed version exists and not even then the behavior has to match your description. There are also a lot unsuffixed mutating methods. See also Kernel#exit vs. Kernel#exit! (immediate exist/exit handlers aren't called) or Tempfile#close vs. Tempfile#close! (close and unlink).
That's not quite the usual statement of the rule: ! is for methods where there exists a "dangerous" version and a "less dangerous" version, to denote the more dangerous version. A common example of that is methods where one version returns a copy of the original object with some modification applied and the other version modifies the original object in place, but IIRCT that's not the only distinction it is used for in the standard library and, more importantly, in-place mutations that aren't paired with a "return-a-modified-copy" method don't get the !.