For https://fedoraproject.org/wiki/Changes/BtrfsTransparentCompression, every btrfs filesystems created by Anaconda should be mounted with `compress=zstd:1`
This kind of filesystem specific change belongs in Anaconda's kickstart handling code, not upstream.
Per discussion with the Anaconda team, here's the PR adding the ability to set [Storage] btrfs_compression and passing it to Blivet: https://github.com/rhinstaller/anaconda/pull/3177
This is in 20210219 composes (tested with Fedora-Workstation-Live-x86_64-34-20210219.n.0.iso). Mount option 'compress=zstd:1' used for the duration of the installation, and is in /etc/fstab.
# compsize /media/root
Processed 132149 files, 78870 regular extents (84614 refs), 73010 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 52% 2.5G 4.8G 5.4G
none 100% 1.2G 1.2G 1.3G
zstd 36% 1.3G 3.6G 4.1G