In the lead up and after the 2020 presidential election, it was clear that voting by mail had a clear divide, mostly driven by Donald Trump's demonization of the process as well as other Republicans. However, no states (as far as I know) have moved to outright ban mail in voting after the 2020 presidential election. Mail-in vs in person (election-day) votes per 538:
Trump, then the incumbent Republican president even won in-person votes in deep blue states like Hawaii and Maryland as well as a few others. The elections in 2021 so far show that the mail-in vs in-person partisan divide appears to be holding up. Biden won mail-in voting in states like Oklahoma and Arkansas.
Why haven't Republicans actively moved to restrict voting by mail in states that they have control over at the state legislative level, especially in swing states, as of writing?
Doing so would seem like a logical move given the fact that they passed laws trying to restrict voters that are less likely to vote for them. Such a move could be even more effective because while the gap between White and Nonwhite voters is about as large as mail-in vs in-person election-day (at least in 2020 and may be slightly larger), this could knock off more mail-in voters given enough wouldn't want to vote in person if it was the only option, and this would be a far more direct move than say closing a polling place in a heavily Democratic area.
Note: four Republican controlled states, and 3 solidly Republican states, restrict mail in voting and require disability and/or being elderly to use a mail-in ballot. But these are not new restrictions. Also, people asked if this is discrediting. I disagree because Republicans have gone on tape publicly to say not letting certain people (often African-Americans, the most Democratic major constituency of all) vote at as high of a rate would increase their odds. Yes it doesn't say that specifically but studies show Voter ID laws have these effects. See here for evidence of a modest but potentially significant effect of voter ID laws which is a precedent for such motives.