To make this more clear for other users:
No.Currently you cannot use FIDO/U2F on iOS, not in the app, nor to acces the webvault using Safari. WebAuth support from Apple is available and Google implemented this very recently (june 3).
And yes you can use a Yubico key using Yubico OTP but then you have to use one of the only two available OTP slots. If you don’t want that, you can always fall-back to regular TOTP, using Yubico authenticator (which works on iOS over NFC), or any other TOTP app.
I second the request to support FIDO/U2F on iOS. See also here.
Implementing U2F on iOS makes the Yubikey OTP flow kind of obsolete. After all every Yubico OTP key also has U2F (if i’m not mistaken). And of course the many other U2F keys get to function as well.