I do believe that the advertising-funded models definitely are not to be depended on, and social media has suffered a lot because of that. I also think that the enforcement of that responsibility does fall in HR, but maybe DEI is also needed for that. Some of the time you're dealing with civil rights cases and other times you're seeking out diverse, non-traditional candidates to add to your company's perspective. It doesn't even have to be race-specific it could also be class-specific.
That era is over - the majority of unprofitable companies have been purged, what remains makes actual money and is much less exposed to public opinion. Therefore, not only is there much less need to virtue-signal, but said virtue signalling and people behind it is now a major expense deemed unnecessary in what is now a for-profit company whose primary objective is to make profit (and not be a playground). Thus, "DEI" people get the boot.
Is this to say some of the concepts behind DEI are bad or unnecessary? No, but you don'y need "DEI" for that, you just need a competent HR department. It should be HR's job to deal with cases of racism, discrimination or noxious unprofessional conduct.