I would also be interested in having a Librem Key that supports U2F. Yubikey might be the leader here right now for these security keys but I don’t think it’s supported to backup the keys on them.
The Librem Key is based on the Nitrokey Pro. Only the Nitrokey 3 and FIDO2 supports U2F/FIDO2. When I asked Purism support about a year ago about whether or not they will release a new Librem Key based on the Nitrokey 3, they said they did not have plans for it.