A friend used this for trying out Obsidian for a while, although he decided to use something else than Obsidian in the end.
flatpak info --show-permissions md.obsidian.Obsidian
flatpak override --nofilesystem=home md.obsidian.Obsidian
flatpak override --nofilesystem=xdg-run/gnupg md.obsidian.Obsidian
flatpak override --nofilesystem=xdg-run/app/com.discordapp.Discord md.obsidian.Obsidian
flatpak override --nofilesystem=/mnt md.obsidian.Obsidian
flatpak override --nofilesystem=/run/media md.obsidian.Obsidian
flatpak override --nosocket=ssh-auth md.obsidian.Obsidian
flatpak override --no-talk-name=org.freedesktop.secrets md.obsidian.Obsidian
flatpak info --show-permissions md.obsidian.Obsidian