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Discord uses a modified version of electron, and it's also probably an outdated fork as well, although I am not sure about that.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.

Sources:

• ARSTechnica, Chrome's "Manifest V3" plan to limit ad-blocking extensions is delayed, https://arstechnica.com/gadgets/2022/12/chrome-delays-plan-to-limit-ad-blockers-new-timeline-coming-in-march/
• GHacks, Brave confirms it will support Manifest V2 extensions like uBlock Origin even after Chrome drops them, https://www.ghacks.net/2022/09/29/brave-browser-manifest-v2-extensions-after-v3-update/
• Valve Developer Community Wiki, Chromium Embedded Framework, https://developer.valvesoftware.com/wiki/Chromium_Embedded_Framework
• Github, Discord's Electron Fork, https://github.com/discord/electron

If I missed something, please tell me!