It could still work, but you would have to remove/reinsert the key to confirm that you are present. An U2F key with a button saves you the trouble and the wear on the USB port. (I think they are rated for a large number of insertions, but still…)
A touch area, like Yubico use on their keys, would have a lower BOM cost than a push button and would also not wear out. Perhaps that solution is patented, though?
Also, U2F can use RFID at the transport layer, in which case you would tap the key to your phone/device. No button needed. Would be perfect on the LIbrem 5 (but there’s no on-chip RFID support on the i.MX8 IIRC, despite NXP being RFID pioneers ). Would be nice on the Librem laptops, too, but can’t remember seeing any laptop (any brand) with RFID.
I think Bluetooth (LE?) is supported, too, but I’m not 100% sure. That would need a button, though.
Except maybe the Yubikey 4, which does U2F in addition to GPG, OTP, and other stuff.
I’d like to add that a neat thing about U2F is that there is no shared secret between the key and the computer/phone/web service you authenticate to. It uses public key cryptography. Even if the data is stolen from the phone/computer/service the key as such is not compromised.
Not so with TOTP/HOTP… Whoever gets the shared secret from a file or database will be able to clone the key and in effect bypass the 2FA.
Please, please, can we have U2F ?