I think it was Patch 3.1 or 3.2 that had the nerf. this was about the time of the 2020 shut down, so I came across the posts afterwards when things started opening up.
for those who don’t know, Crushing Flames (6) instant kills Normal (grey/white) monsters. With Frozen (5) increasing the damage of the killing blow by a +125% explosion, with enough Normal monsters near an Epic+ monster, it would be possible to kill them with just the Frozen Explosions.
this is the earliest mention of the Crushing Flames & Frozen Set combo nerf and another post where it’s mentioned.
I don’t use Frozen Set, and I think the one time I tested it, it didn’t really fit my play style. my conclusion on the subject is that, if there is a nerf, it is because only Crushing Flames Set affects the damage of Crushing Blow, and Frozen can’t take advantage of the instant kill of Crushing Flames (6) on Normal monsters anymore.
there is an older post where a Dev explains why Crushing Blow isn’t affected by any other affixes, and I think this was before Crushing Flames came out.