A detachable magazine is not 'a' feature - it's one of the defining features.
A semi-automatic rifle made/assembled after 9/13/94, with the capability to accept a detachable magazine may have at most ONE of the following:
pistol grip,
folding/collapsing stock
bayonet lug
grenade launcher
flash suppressor/threads for a flash suppressor
If a post-ban rifle has TWO listed features, it's an illegal 'semi-auto assault weapon'
The standard post-ban AR is a semi-automatic rifle with the ability to accept a detachable magazine, so its usual ONE feature is the pistol grip. If you remove the pistol grip, you can have a collapsing stock.
If you make the magazine permanently non-detachable, you can put anything on it you like - including the flash suppressor and collapsing stock and bayonet lug. Some state laws (like California) are more restrictive, and won't allow the pistol grip or flash suppressor regardless of magazine status.