Btrfs with kernels >=4.14 support zstd compression. However, Grub does not yet support zstd, which leads to failed boots if zstd is used on the boot/root filesystems. Support is being added at https://github.com/kdave/grub Perhaps a warning when installing grub and/or btrfs-tools about the incompatibility would be good?
Feel free to update the wiki with a warning, but I'm not going to check for this in the ebuild.
