Uninstall

From each host

Stop your sessions first (simstim ls, then simstim kill <host>/<session>). Then, on the host:

systemctl --user disable --now simstim-restore.service
rm -f ~/.config/systemd/user/simstim-restore.service
rm -rf ~/.local/share/simstim ~/.local/state/simstim

Linger, which host add turned on, stays on; loginctl disable-linger turns it off if nothing else needs it.

From the laptop

rm -f ~/.local/bin/simstim
rm -rf ~/.config/simstim ~/.cache/simstim ~/.local/state/simstim

The client never edited ~/.ssh/config, so there is nothing to undo there.