This isn’t technically correct when Flatpak is installed via your distribution. Then it is managed by your packaging system and it’s data should live in a /var/lib.
/opt is for software that doesn’t adhere to the FHS and can’t/shouldn’t be installed in a prefix. Red Hat and Arch install software to /opt via their official package managers. /opt isn’t safe to stick arbitrary software even though it’s commonly used for that — /opt/local is the non-FHS equivalent of /usr/local.
> This isn’t technically correct when Flatpak is installed via your distribution. Then it is managed by your packaging system and it’s data should live in a /var/lib.
Besides, this is silly: they go in /var/lib/flatpak, which is for flatpak's program state information. (Exported desktop files and whatnot go to /var/lib/flatpak/exports/, which is pointed to by `$XDG_DATA_DIRS`).
I mean, if you want to go all Militant Unix Admin on it and install apps to /opt, you can `man flatpak` and set `FLATPAK_SYSTEM_DIR=/opt/flatpak`, but unless your system spans a dozen hard drives I'm not sure what that's going to achieve.
I guess this is sort of an ownership conundrum: a flatpak app is run _via_ flatpak. On its own, it's just a heap of files.
