Reelhost records what it changes, with the setting it replaced, on this computer. Undo next to an item puts those settings back.
Add-ons Reelhost installed stay installed but are turned off; Jellyfin unloads them the next time it restarts. Remove them under Plugins in Jellyfin's dashboard if you want them gone.
Removing Reelhost from this computer leaves these settings and add-ons as they are, so watching keeps working. Undo them first if you'd rather not keep them.