Hi Matwey,
Currently, half of JeOS images are supplied with ext4 and the other half - btrfs. And frankly speaking, I cannot figure out the rule why some specific board receives JeOS with ext4 instead of btrfs or vice-versa.
The idea is to move to btrfs if the particular device supports that filesystem as root device. Note however that btrfs is particularly unstable on aarch64, so it isn't a particularly great choice to use, but hopefully those issues will get fixed in the future by the filesystem team.
I suppose this was related somehow to btrfs support in u-boot, right?
Yes, ext4 is/was the historic choice (have something that works, similar to how android works).
Another consideration. Does anybody know any research on what FS is the most friendly to SD cards?
I am not aware of such research, but I would guess F2FS is very high on the list. btrfs is slightly better than ext4 in that regard. Typically it helps here to follow the Android lead (they switched from ext4 to F2FS years ago). Greetings, Dirk