When copper armor was added to the tiers of armor zombies and skeletons can spawn with, the chances for them to spawn gold and higher tiers of armor were reduced by a large amount (gold by ~4x, chainmail by ~5x, iron by ~8x, and diamond by ~10x). This is due to the way copper armor was added to the pool of mob armor tiers: all tiers gold and higher were shifted up by one tier, copper was added as tier 1 (above leather (tier 0)), and a 9.5% chance was added to increase the tier by 1. This adversely affects the odds of gold and higher tiers of mob armor appearing, since prior to 25w31a, the game generated a random integer between 0 and 1, then rolled a 9.5% chance to increase the tier by 1 three times; now this is done four times, requiring each tier above leather to roll an additional 9.5% chance to be selected compared to before.
The recommended fix is to remove the fourth 9.5% chance to increase the armor tier by 1, and instead have the initial roll select a random integer between 0 and 2. This would still leave the overall odds of other armor types appearing lower than before, but only around 33% lower instead of several times lower.
Can confirm, with the local difficulty at the highest I see a lot of copper and leather armored mobs, but gold and iron armored mobs were much less frequent and a pretty rare sight.