TLDR; Farming old content now is almost impossible to do solo because of changes to damage
As the expansions progressed, player damage had become an issue. WoD saw a reduction in player damage numbers and so did Shadowlands with the level crunch. This caused some discontinuity when players went back through old raids/dungeons, where players still did that massive damage.
It appears that one of the unmentioned changes in 9.1 was a re-alignment across all content to match the damage that players do in Shadowlands. As I was just in Molten Core, i was struggling to solo the third boss. I haven’t tried later expansions yet but I imagine this will likely be worse in WOTLK+ raids.
Big sad