I would also look for those having sold “naked call options” to see a margin call.
Over all affect to the financial markets may be a zero sum gain or loss though as so many instruments are “insured” to make the net result a near zero if they have to be sold.
Just IMO of course, not by any means an expert or even an amature in such things.
If you sell a naked call option and it is in the black this rule won’t change anything. Margin only kicks in when the option is losing money.
My gut feel is that this will have some effect on short stock positions that might get called in by the margin call; that would mean some upward pressure on prices.