if your service/dynamic user can only write to some very specific private directories, you can just delete those and be sure nothing of that dynamic user will persist.
So, what are those "capsules" then, that this story was all about? Well, it combines the concept of the per-user service manager and the concept of DynamicUser= into one thing: there's now a new template service capsule@.service which is very similar to user@.service. The latter wraps the per-user service manager…
… for regular users. The former is very similar: capsule@.service can be instantiated many times, and each will spawn a per-user service manager too – but has DynamicUser=1 set, i.e. it's a service manager for a dynamic, short-lived user.
So, what is this good for? The idea is that you place some service definition files in the .config/systemd/user/ subdir of /var/lib/capsules/<somename>/. The latter becomes the $HOME of the dynamic user created for capsule@.service.
With that in place, …