Had spacesuits been used in TAS, fishbowl helmets and color variations by division should have been more than enough to make the characters recognizable and distinguishable. The point that the diving suits in Sealab 2020 worked well enough is all the proof needed to show that spacesuits could have worked adequately in TAS.
Sure, they could have, but that doesn't mean that character recognizability wasn't one of
the factors that led to the decision to use force field belts instead. I never meant to suggest that the belts were the only possible result of that consideration -- just that it was probably one
of the reasons behind the decision to use the belts, reasons which also included simplicity, economy, and perhaps the desire to take advantage of the freedom of animation to depict something more futuristic than TOS could've done. Although of course simplicity was probably the primary consideration.
Additionally, the point that continuity issues with later shows exist only because the later shows failed to use something similar to the belts is a good one, I think.
Might be worth mentioning that a DC Comics Star Trek
annual written by George Takei and Peter David did have characters in the Enterprise
-A era use force field belts of a sort, although they were for biohazard protection rather than taking the place of a spacesuit.