Off the top of your head, no cheating:
- Name the last episode of telly Doctor Who that mentioned the regeneration limit.
- Name the last episode of telly Doctor Who before that that mentioned the regeneration limit.
1) Mawdryn Undead
2) The Deadly Assassin
(Unless SJA counts as televised Who, then Death Of The Doctor sneaks in)
And, unless the TV movie counts, for which the whole plot rested entirely on the regeneration limit. (And I recently rewatched it and they mention it over and over again so that views unfamiliar with Doctor Who will understand.)
But yeah, no, it was only a random, throwaway comment.
I don’t mind them fudging their way out of it. I just want them to at least bother to give a reason.
Also: Mawdryn Undead is one of the most interesting serials of Classic Who, and it would be a crime to just pretend it didn’t happen.