I'd bet that by Dropbox's user metrics "the most popular distro" still qualifies as "uncommon" for a Dropbox client. Despite being the year of desktop Linux, non-server Linux boxes are comparatively rare.
That said, Ext4 is still supported, just not with encryption.
No stacked encryption on top of ext4. LUKS+ext4 should still work (since it's visible to Dropbox as just ext4 with cryptsetup/LUKS handling the underling container).
> Ext4 is still supported, just not with encryption.
It's a terrible idea to not turn on hard drive encryption. Anyone who gets their hands on the machine can read and write to the drive by booting it with a thumb drive.
That said, Ext4 is still supported, just not with encryption.