"All raid bosses should be able to mitigate heroic effects more equally. Previously some bosses were much weaker than their counterparts." As stated in the patch notes, what are heroic effects?
I think the advantages gained from heroic stats would make old bosses more trivial because it would raise caps to the point the boss was unable to inflict damage or dodge an attack? This is probably more of a TLP thing.
There were apparently a couple of old raid bosses in TDS and TBM that if you had over 1900 dex or agi, the mob could not hit you with melee, because their strikethrough was so low and your avoidance was so high. They bumped them up to be similar to other bosses in the expansion. Enjoy TLP servers, if you ever get that far!