Yes, my initial evaluation was flawed because I was looking at the free version of Bitwarden, but supports neither U2F nor attachments, but the evaluation grid said that it didn't support U2F but did support attachments. I've updated the grid to fix this. It now says that YubiKey is supported for Bitwarden and has a separate pricing line for personal use without attachments or YubiKey vs. with them.
"Yubikey support" is a meaningless phrase, anyway.
Bitwarden supports 2FA with Yubico OTP - although there's a bug so it works only for QWERTY layouts. Or you can use Yubikey's static password feature for your master password, I guess.
There's also OpenPGP Card and PIV, which, to my knowledge, is not used/supported by any password manager software except for `pass` and some compatible implementations.
U2f support is badly hampered by half-assed browser support. Only chrome enables it by default, Firefox disables it by default, and no love from safari. Even LastPass in the browser uses yubico’s proprietary otp algorithm rather than u2f.
Google and Github both built their U2F support for Firefox before WebAuthn was released, and as you've pointed out, the U2F support in Firefox is gated out by default. Presumably Google, Github, and other companies that coded to U2F will migrate to WebAuthn eventually.