The user-${UID}.slice units are not templatable, and use a naming scheme that is inconsistent with the rest of systemd.
I particular, this makes it impossible to set per-user ressources limits: @poettering suggested in #2305 to set those in user.slice, but this would set a limit on all users collectively (i.e. “users can use at most 500MiB of RAM in total” vs. “any given user can use at most 500MiB of RAM”).

I understand this is a difficult-to-handle bug report, since simply changing to [email protected] is compatibility-breaking, and some code has user-${UID}.slice hardcoded-in.
However, it should be doable to make this configurable - for instance by honoring the Slice parameter in [email protected] if it is present - in order to support templating and not break backwards compatibility.

Should this be an acceptable solution, I would happily try to contribute patches for this. Otherwise, please suggest a possible solution, so that we can move towards resolving this issue.

Note: This is a rehash of #2305, which was closed because it contained a non-helpful description and has a problematic suggested solution.
I opened a new issue since none of the comments in it were addressed after it was closed, and I was unable to get it reopened.