MIT is MIT. It already had that policy. MIT can tell who the top, brilliant math/tech kids are without subject scores (I’m guessing the majority of apps they were getting had 800 subject tests before they dropped them, so that didn’t differentiate kids). There are the AIME, ISEF, olympiads, etc.
IF an applicant to most colleges can still take and get high subject test scores, they will be another piece of the application. I think for 2021, even colleges where kids normally submit them won’t expect them though. Some kids may not be able to take them by ED or EA.