This should be the commit that introduced the warning:
https://github.com/systemd/systemd/commit/bed0b7dfc0070e920d00c89d9a4fd4db8d974cf0
Based on reading it, setting `DynamicUser=yes` seems to be what systemd wants
you to do.
Official documentation for `DynamicUser`:
https://www.freedesktop.org/software/systemd/man/systemd.exec.html#DynamicUser=
