Chromium had it for qhite a while, but it isn't really relevant... Discord's implementation of screen sharing was custom on X11, if they had used the one that comes with Electron, this would've worked far earlier.

operating systems may not have implemented it umtil more recently

DEs that had a Wayland session (aka Gnome and Plasma) supported it very soon after the portal was made.

The real reason won't be anything external, but something in the company. Usually it's just that Linux isn't a priority for a given company, so even if there's a motivated engineer that wants to take care of it, it's hard justifying to their managers why they need to spend a lot of time on it.

This isn't exclusive to Discord, to use a very similar example, Zoom is kinda worse. In the past, Zoom misused a Gnome screenshot API to do screen casting very badly, and recently they ported to the desktop portal - not because they had a choice, but because Gnome locked down the API they were using. Screen casting still only works on Gnome though, because they still check for the desktop name. If you set it to Gnome, it works perfectly fine everywhere else too!

All it would take to fix that problem is removing an if statement, yet, despite many complaints, it hasn't happened... because no big customer has complained, so it's just one of the unimportant Linux bugs.