When creating a backup of a directory, I would like to exclude directories that are included via a tmpfs mount. I have seen a lot of path-based exclude options, but this would make configuration still too explicit.
Something like find’s -xdev parameter that does not go into subdirectories that are on other file systems.
There is a -x aka --one-file-system flag available in the backup command: exclude other file systems, don't cross filesystem boundaries and subvolumes.
That was a fast response 
I did not see that. This is exactly what I was asking for. Thank you!
Just to say, the “exclude other file systems” of this flag means, it will not include Any other mounted filesystem into the backup.
That was something I missunderstood years ago when I started with restic. I used the flag because I also didn’t want to exclude proc, sys and others by hand. But I was not aware that it would also ignore my home partition, which was mounted through another partition.