

It’s more that compositors (picom, xcompmgr) and window managers are extremely complex, and adding that feature in could have much larger impacts than just the functionality itself, e.g. security context implications, task scheduling, rendering pipeline issues, etc. Wayland actually combines the compositor and WM, so it’s more readily extensible than X11.
No, this is different. OpenCore refers to when part of an application is open source, but major functionality or components of the application itself are not.
ElasticSearch is this way, where the elasticsearch package itself is fully open source (AGPL), but you have to pay tens or hundreds of thousands of dollars for its SIEM addon.
What OP is talking about wrt discord is not actually non-FOSS at all, it’s purely about the community (support, discussions, and documentation), nothing about the application itself.
Even the Steam integration does not change the status of the source code itself; OP could fork it and remove the Steam integration.