Ran into this while walking a friend through installing CachyOS, they're coming in from Windows with fresh eyes so I figured I'd take notes on all the little weird things they ran into. Compiling an entire web browser from source is pretty unnecessary, and ideally it would also be installing the dependency for the web store to work as well as someone relying on the GUI would expect to be able to use extensions and would not think to go looking for that dependency.
It looks like PDFMod, Whatsapp web, and Slack web aren't available in any repos anymore and can't actually be installed. gtkspell, pidgin-gtkui, hexchat, backintime, simplescreenrecorder, deadbeef, xplayer-plparser, xplayer, sdl2_pango, tuxpaint, pinta, ms-365-electron-bin, opera, and midori are also installing from the AUR, but they're not available in the cachyOS repos so that's to be expected.