EXIT STATUS
Normally the exit status is 0 if a line is selected, 1 if no lines were
selected, and 2 if an error occurred. However, if the -q or --quiet or
--silent is used and a line is selected, the exit status is 0 even if
an error occurred.

is it better to use --silent?

-q, --quiet, and --silent are the same flag, and no that's not a suitable option:

-q, --quiet, --silent
Quiet; do not write anything to standard output. Exit
immediately with zero status if any match is found, even if
an error was detected. Also see the -s or --no-messages
option.

also we're not trying to swallow errors, we're remapping from:

• match: 0
• no-match: 1
• error: 2

to:

• match: 0
• no-match: 0
• error: non-zero

ahh

TODO: We could consider using a less terse version and retain the exact exit code.
right now that's not important and I'd rather not have to rebase and re-build #2737 again unless there's a seriours error here.

We probably should follow-up with preserving the exact status for future usage.

Or just switch to pure awk.

Yeah, it seems that awk would look nicer. Something like

unsupported_cgroups=$(findmnt -lun -o source,target -t cgroup | awk '!/'"${current_cgroup}"'/ {print $2}')

It feels slightly wrong that we're regex matching in either case instead of substring literal but :this-is-fine:
I suppose we could regex escape it with some more bash 🙈

It looks like if we stuck with grep we could use -F / --fix-strings.

For awk, one can use something index(string, substring) == -1. Some implementations treat substr as a regexp, but GNU awk does a fixed substring match.