From Wikipedia: Universal 2nd Factor (U2F) is an open standard that strengthens and simplifies two-factor authentication (2FA) using specialized Universal Serial Bus (USB) or near-field communication (NFC) devices based on similar security technology found in smart cards.
Using Yubikey to mean U2F is like people saying "Google this term", "the image is Photoshopped", "Hoover the floor", "grab me a Kleenex", or even "take the escalator". It possible "Yubikey" could become a generic trademark, but if possible people should be wary of using brand names in this way before it 'sticks'.
Neither Photoshop, Kleenex, nor Google have lost trademark protection.
Additionally, the term Yubikey isn't likely to become synonymous with 2FA in any case. Most people don't know that yubikeys work in several different, independent modes, such as FIDO/U2F 2FA, or CCID smartcard, or Yubico OTP (those long annoying strings your yubikey types when it brushes your thigh or hand).
The CCID smartcard mode requires a pin, which is technically two factor authentication (knowledge of PIN and possession of yubikey), which is an entirely different thing than FIDO/U2F 2FA (which is what most people mean when they talk about using a yubikey for 2FA, not that "yubikey" and "2FA" are interchangeable terms).
This is further complicated by the fact that CCID smartcard mode can be used for ssh (via gpg-agent, with ssh keys inside the yubikey itself), AND, separately, OpenSSH (with other keys) can use a yubikey for U2F.
