I can confirm that helmets are restricted by proficiency. But in 5E D&D, helmets aren't a separate thing -- they're part of armor. In BG3, they give benefits (like the super-amazing protection from critical hits). I don't have a strong feeling about this either way, really.
That helmet benefit (protection from critical hits) is straight up from BG1/BG2, all the basic helmet gave it to you.
