TheWoWDBDurotan will stay in your Garrison (after questing to a certain point) as long as you don't complete all Frostfire Ridge story quests. Ga'nar will also stay if you did it to the point where you helped kill sons of the Iron Wolf.
Edit: Just upgraded my Garrison to level 3, Durotan&Thrall&Ga'nar still there.
Elite
Durotan
Details
This roster comes from the current game client's data files, so it is not a complete list of every NPC in the game - older world NPCs that are no longer in the modern client are absent.
Community notes from Wowhead · 1 comment
The comments below were written by Wowhead users. They are community notes, not TheWoWDB comments. See them in context on Wowhead.
Level and Map Tracking
The game data tunes Durotan for levels 10 to 40, Warlords of Draenor content.