systemd-repart definitely does, but requires GPT. It will not repartition MBR.

cloud-utils-growpart doesn't have anything btrfs specific in it, but all it ordinarily needs is for something else to properly do a 4KiB aligned partition extend; and then btrfs filesystem 1:max /and the fs resized, which effort wise amounts to writing new superblocks, that's it.

So... question is if the GPT proposal for GRUB wants to switch to systemd-repart? Or keep using cloud-utils-growpart?

Well, switching to systemd-repart looks like it requires a bunch of configuration? I'm not sure about it. Maybe @zbyszek could give some detail here, because I don't understand the tool well.

As for cloud-utils-growpart, it seems we use this with cloud-init. with its growpart and resizefs modules, which include Btrfs support: https://github.com/canonical/cloud-init/blob/main/cloudinit/config/cc_resizefs.py

So at least for now, we don't need to do anything.