Steam uses the Chromium embedded framework in case anyone doesn't know. This renders the web pages in the Steam client. As mentioned, there's no point in Valve maintaining the code base themselves when upstream Chromium drops support for 7.
This is similar to when browsers dropped support for Flash. Adobe stopped developing it and the major browser vendors removed their in-house flash plugins.
Except it's not perfect for gaming. If you happen to have titles purchased through the Xbox/MS storefronts, you won't be able to play them. The version of windows you speak of lacks three critical system packages that allow UWP based games to work. Xbox Identity Provider, TCUI, and speech to text (some games rely on that for accessibility). If you file any bug report or ask for support from the development, they'll discard your ticket when they look at logs (unsupported OS). You also gimp yourself on feature sets.