I have seen some sites that only show a username field but nevertheless are compatible with my password manager (1Password). What I suspect is going on is that the password field is present but visually hidden (not missing and probably not with display: none, but perhaps by being placed in an overflow: hidden box with a height of zero or a similar technique).

I haven't confirmed this hunch as to the technique but it seems like a good compromise if there is a good reason to hide the password field initially. And I think there are some such good reasons. For example, if you are Google: Not everyone logs in to Google with a password. I have to log in to my work account using our company's SSO provider, so that Google account has no password. In this case, I shouldn't see a password field, as its presence will be more confusing than helpful. Still, a hidden-but-present password field would allow my password manager to work in the case that my Google account does in fact take a password. (Presumably care should be taken to avoid adding extra confusion to users of assistive technology.)