Used. Only the orignal Jolla phone introduced at the end of 2013 used btrfs. There have been many many problems with btrfs on the device, mainly due to metadata vs data block allocation issues & resulting metadata space exhaustion. The balance ooeration being basically unbounded in time and fregvently crashing outright did not help.

Since then all other devices made or officially supported by Jolla are using EXT4 on LVM, including this one Xperia X running Sailfish OS I'm typing this on.

Huh, I could have sword the Xperia X port also used BTRFS, but I guess I never checked.

(And yeah, those issues were how I found out: unable to create files while having "4 gb of free space" - not helped by the fact that it didn't run the balancer automatically.)