Python 3.9 (and 3.10) support has been committed and released. I don't think your Debian instructions are correct; iptables-legacy is the old iptables, not nftables. nftables provides iptables-nft which supports the same syntax but bases it on the new kernel interfaces. Regardless I think either should work for netfilterqueue as the queueing capability is core to Netfilter and is not dependent on which firewall is used. Both alternatives of iptables provide the same interface and thus should be equivalent for our purposes. It is also possible to express queueing using the new native nft tool: https://wiki.nftables.org/wiki-nftables/index.php/Queueing_to_userspace

Are you seeing a difference in behavior on new Debian or did you just want to add the nftables info for completeness?