Compressed with `zstdmt`.
To uncompress, install zsdt (sudo apt install zstd), and then run:
tar -I zstd -xvf RADIXDB-INDEX.tar.zst --exclude=./address_book -C ./output-dir/
(old:)
tar --use-compress-program=zstdmt -xvf RADIXDB-INDEX.tar.zst --exclude=./address_book -C ./output-dir/