The following experiment (see video) shows that skeletons no longer counterattack in 1.21 experimental.

1. Turn on "Update 1.21"
2. /set time night
3. Summon two skeletons and one Iron Golem in the fense
4. Create another world, whose "Update 1.21" is off and repeat the experiment.

In step 3, skeletons won't counterattack.

In step 4, skeletons will counterattack.

Expected Results: Skeletons will counterattack.

I also test skeleton accidentally damages zombie, and the zombie also doesn't counterattack. Therefore, multiple mobs are affected.

Note: This phenomenon only takes place in 1.20.70.21 or after with "Update 1.21" on, and doesn't take place in 1.20.70.20 or lower.