So its a slightly worse version of feeblemind because there is now no reduction to INT and CHA scores, using magic items, Understanding language or even communicating properly.

However it deals more damage instead of 4d6. 

So the new version is more for combats against magic casters. whereas the previous was a blocker for all classes and uses of magic items but less damage. 

This one doesn't allow the Magic Action, which includes all magic items and spell scrolls.  It basically does the same thing as before, but doesn't affect saves, skills or language.  Still a nerf, but then more damage, so...

why is it avaiable to "Legacy Test Subclass", a barbarian subclass?

The problem with the old spell was that it made your intelligence 1, and then you had to succeed a potentially dc 16 saving throw or higher with a negative 5 to your intelligence modifier. This meant that without bonuses (and even with bonuses if the dc is from a high level caster, and it will be since they can cast this 8th level spell) it could be literally impossible to ever get rid of the spell without greater restoration, dispel magic, or some other remedy. In the mean time, you had to roleplay being a completely incapable character, and if your character was known for being charismatic or intelligent, you can say goodbye to any of that. It's still a very oppressive spell worthy of being 8th level, but it's nice that now it doesn't completely lobomotize your character, it just prevents using magic and does some good psychic damage.

Honestly I think the changes are to encourage it actually being taken by players. Pretty much every DM I’ve played with has banned the spell, and I can understand why.