From systemd.exec(5):

—

ProcSubset=

Takes one of "all" (the default) and "pid". If "pid", all files and directories not directly associated with process management and introspection are made invisible in the /proc/ file system configured for the unit's processes. This controls the "subset=" mount option of the "procfs" instance for the unit. For further details see The /proc Filesystem². Note that Linux exposes various kernel APIs via /proc/, which are made unavailable with this setting. Since these APIs are used frequently this option is useful only in a few, specific cases, and is not suitable for most non-trivial programs.

2. The /proc Filesystem: <https://docs.kernel.org/filesystems/proc.html#mount-options>