Brave, Vivaldi, Edge and other chromium browsers are forks of the main chromium project. They can decide whether to include or exclude features from mainstream chromium.
As far as I know, Brave and Vivaldi will keep Manifest V2 extension support and said that they will not ship WEI (Web Environment Integrity).
Discord uses a modified version of electron, and it’s also probably an outdated fork as well, although I am not sure about that.
Steam, in the other hand, uses CEF, which they use as a way to render it’s interface and as a replacement of VGUI (a good example of this is the steam game overlay), I don’t know if they will ship WEI if it ever releases in chromium as there isn’t a statement from Valve yet.