I've had a few people recently claim that squadrons can't use accuracy results against ships (usually where one of my B-Wings rolls a blue accuracy and a black hit).

Is this correct? I can't seem to find any such restriction in the rules.

Similarly, if my one of my anti squadron dice (from other squadrons or ships) comes up with an accuracy result, could I counter (say) Howlrunner's scatter token?

You can absolutely use your accuracy icons in Squadron Vs Squadron combat or Ship on Squadron combat.

Accuracy can ALWAYS be used.

Maybe what they meant is that Squadrons can't use accuracy because by the time squadrons get a chance to attack, all the defense tokens have been used up (unless you're using squadron commands). Generally, if I'm getting bombarded by other ships, I'm most likely going to use my defense tokens well before any squadrons attack.

You can still accuracy an exhausted defense token

They can use accuracy, the problem is it usually doesnt help.

Majority of squadrons only have 1 die against a Ship, so it might as well be a miss.

Only named squadrons have defensive tokens, so unless youre firing multiple dice against a named squadron it might as well be a miss as well.

Its not that they cant use it, its that theres almost no situation where they CAN use it due to dice counts.