With the pandemic going on, most schools have either decided to make both GRE and its subject tests optional. In all likelihood, would there be a big chance that this is the finishing hammer for the GRE, and make a lot of institutions move away from it?
My U.S. grad program in math had already been reducing attention to GRE subject test for years, due to its manifest uselessness for predicting success in our grad program. (I've been involved in grad admissions here almost continuously since about 1985, and have been Dir of Grad Studies in math for two stints, so I've really paid a lot of attention to this.)
Until the pandemic, we did tell our own undergrads to take the subject test GRE, since many places still did expect it. In fact, faux-ironically, until recently, we did tend to wonder why someone would not have taken the subject test GRE if they had any idea what they were doing... in terms of preparation for or application to grad school in math in the U.S.
A number of other good programs had already, in recent years, downplayed the significance of the GRE subject test score for their applications, so we ourselves had come to "wonder less" why someone might not have taken it.
With the pandemic, and the at-least-temporary vanishing of the GRE subject test, we have more-unabashedly stated that we don't care about a subject test GRE (nor any other part of the GRE).
I would speculate that other math grad admissions in the U.S. have also figured out how to not pretend that the subject test score is a good predictor of success... all the more that it (documentably, both anecdotally and other) never was, and the last year or so has necessitated making decisions without it.
This will vary by institution in the US. Some will return, others not. It is likely to depend on how the experience of making predictions about success of applicants turns out without it. If the other indicators prove adequate there will be little pressure to return.
But, I suspect that many, if not most, will return to using it since, for the institution, it give some information at little cost or effort.
Hard to predict the future, of course.