Needing xdg-desktop-portal may be in my future (even without Wayland)
I generally haven't had particularly positive experiences with xdg-desktop-portal on my custom desktop. At best it appears to do nothing; at worst, it's been a core element in mysterious problems. Having xdg-desktop-portal installed effectively isn't optional these days on Fedora (if you try to deinstall it, it takes out a bunch of other things, although in Fedora 36 there's fewer things than I expected). However, letting the program and its whole portal environment do anything so far seems to be optional unless you use flatpaks.
(To my surprise, our Ubuntu 22.04 systems don't seem to have xdg-desktop-portal installed at all, despite having a bunch of graphical things. The clue may be in what dnf lists as depending on xdg-desktop-portal on my Fedora 36 system, which is mostly gdm, gnome-shell, and gnome session things.)
Recently I read Artemis Everfree's An X11 Apologist Tries Wayland (via). One of the things I learned from the article is that on Wayland, a variety of ordinary programs (running outside flatpaks) rely on xdg-desktop-portal and its friends to do a variety of desktop operations; screen sharing is the example mentioned in the article. This makes a lot of sense, since in Wayland this sort of thing requires additional, non-standard interaction with whatever Wayland compositor you're using, and as I understand it GNOME and KDE have gone their own way on compositors.
One of my personal rules of thumb for open source is that open source programs tend to converge on having only one way of doing annoying things. Following this, if you have to support xdg-desktop-portal's DBus based stuff to do things on Wayland anyway, it's probably going to be tempting to only do these things through x-d-p even when running on X11 (provided that x-d-p supports them on X11, and that the environments you want to run in have a recent enough version of x-d-p). As a result, I suspect that sooner or later, getting a functional x-d-p environment may be something I need even on X11. Hopefully by that time there will be a 'generic X11 window manager environment' backend of xdg-desktop-portal (which would be sort of analogous to the wlroots one for Wayland).
PS: of course another option is that programs migrate to requiring x-d-p and no one writes a generic X11 x-d-p backend, because no one cares enough and everyone is theoretically moving to Wayland anyway (the practical side is another issue). But I can hope that one doesn't happen.