That'll be hard, especially since the the Mantle spell is essentially an "I win" button, evolving with the caster from levels 12-30 eventually giving immunity to anti-magic fields, wish and time stop, unlimited use Fly, Shapechange and Telekinesis, selecting an entire school of spells that bounce back on their caster when they target you, maximizing and protecting you from 1 spell per level over 12th, and taking 9 successful dispel attempts before going down, all in a 6th level spell.
Okay, so that's probably not the main thing they're looking for, but point-by-point:
*Antimagic is a very situational tool already; taking it off the table is probably fine. The only thing that can't easily be emulated by other effects is its' potential as a general hammer, and tool denial should not be used as a general hammer.
*In combination with immunity to Wish and Time Stop, it sounds like the writer was ticked at some GM(s) for using those as railroading tools/You Lose buttons. Probably droppable, given that the 5th Edition versions of those spells aren't so nasty.
*Unlimited flight is available to Aaracockra from level 1. This is either super broken, not-great-not-terrible, or basically useless depending on the DM's level design.
*Unlimited HP is an issue, although it's not like 5E's Druid 20 doesn't have the same problem. Still, that can probably be jettisoned without losing too much of the shapeshifting feature.
*At-will TK seems like it'd probably be fine, in a campaign with cheap flight. It doesn't do
that much damage.
*School and spell immunities doesn't sound terribly reasonable, no. But on the upside, they can probably be dropped without issue.
*9 dispel attempts
is probably a bit much, if it's something that's ever supposed to be dispellable. At least in 5E's action economy. Three sounds reasonable.
*All told, that's basically Shapechange+Telekinesis+AMF immunity. Most of which is already available to Shapechange. If you say the Shapechange effect isn't available unless you cast it with a 9th level slot, it sounds pretty reasonable actually.
Also, the number of features makes me suspect the Mantle is actually more subclass than spell. So comparing it to other 12-30 classes might make it look more reasonable?