It does somewhat renew itself due to alpha decay, but that probably isn’t fast enough to matter.

Not quite - even in PowerShell 7 there are some features that only work on Windows and Windows only comes with PowerShell 5.1 by default.

If /u/user posts something to their Reddit profile, it’s actually posted to /r/u_user.

Normally this isn’t visible, but if a user account is deleted their profile sub might still exist.