Catwoman in The Batman will be black, but that doesn’t mean all Catwoman appearances from now on will be black. Ditto for James Gordon who is also black in The Batman. Would some people be upset about a white Iris or Wally in a future Flash movie/cartoon/whatever? Sure. Are those people numerous enough to matter? Doubtful. Twitter is a microcosm in the grand scheme of things and the number of people who wouldn’t go see a movie because Iris wasn’t black is not all that high. MCU MJ is black and the MCU is massively popular but they didn’t make MJ in the Spidey games black or in the comics. Tim Drake will be black in the Titans show but I doubt that’s going to translate into him being black in future adaptions consistently. Especially if they do plan to push the multiverse it means that the audience is going to understand that things can differ radically within a “multiverse”.
Typically the characters who are permanently changed are the C-Listers. Take Hawkman: With the reincarnation angle I could easily see Carter being black across the board going forward if Black Adam is a success. He’s got a small fanbase but the mainstream audience doesn’t know him at all really. Hawkgirl has been Hispanic/Latino coded pretty heavily since the DCAU and the New 52/Rebirth made it official. While Bendis’ Legion run may have been a flop I expect the changes he made to those characters in terms of diversity will also stick since the Legion is too big in team size and too small in importance to be as lily white as it was.
But for someone who was as huge as Wally I don’t think it’s a given he will always be black going forward.