
After studying the arcane arts, you have learned how to best eliminate those who abuse its power. You gain the following:
- When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
- When a creature within 120 feet of you casts a spell, you can use your reaction to make an attack against that creature, if you are able. You may use this ability a number of times equal to your proficiency bonus. If the attack you use is a spell, it must be a cantrip.
- When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
- You have advantage on saving throws against spells cast by creatures within 5 feet of you.
- You may cast Counterspell once per short rest, at a range of 30ft.

I think the point of the original mage slayer feat was to give martials a way to at least sort of compete against spellcasters. Making this a caster feat takes away the original purpose.
The feat says nothing about requiring a spellcasting class and because it states "an attack" instead of requiring you to use a "spell attack" that leaves the feat open to be used by non-casters as well as casters. So unless you are assuming only spellcasters can study the arcane (something which is just not true) I simply do not see how you came to the conclusion you made. Sure it gives counterspell but because of the way it works (not needing spell slots and resetting on a short rest) it is more like an ability or an item effect rather than a spell (the same way some races can cast invisibility on themselves for free without needing spells or how non-casters can still use a Wand of Magic Missiles). As for if a counterspell or anti-magic field would stop the ability of the counterspell you get from here since it can be seen as more of an ability than a spell I would leave that to DM discretion (I would personally treat it as if it was cast using an item and as such those things would still have an effect). As a matter of fact, I would argue that this one is better for non-casters because it will allow you to make the attack at a range if you have a ranged char (like a rouge who throws daggers for example). I simply can not understand how you came to the conclusion that this is/was a caster-only feat despite racking my brain for 10+ minutes (perhaps there was a version posted before this which was changed after hearing your feedback but the site usually shows if there were previous versions and I do not see that anywhere). Regardless what the case it is clearly not a caster only feat now (if it was before) and thus your point is moot.