I don't think think he ever lost shamanic powers, as far as I remember in enhancement shaman legendary intro quest it's implied that he has some personal problems and that it makes it hard for him to use shaman powers (but doesn't say he can't use them completely forever).
So since he has always been a warrior as well as a shaman he went back to warrior path while his shamanic powers were dormant.
In shadowlands I think the whole point of Draka quest chain in 9.1 was to show that Thrall has overcome his doubts/personal issues and shortly after in Sylvanas fight we see him being confident enough to use his full shamanic potential again.