Funny, the original commit message for that suggests it was simply a precaution. It's not out of the ordinary to avoid newer kernel features just in case.

> This is a short-term patch. Unprivileged use of CLONE_NEWUSER is certainly an intended feature of user namespaces. However for at least saucy we want to make sure that, if any security issues are found, we have a fail-safe.

from: https://web.archive.org/web/20211022013829/https://kernel.ub...

I really don't get that: having to run something substantial as root seems a much bigger security concern, than what it is shielding from user space (example: hosting a web server at port 80)