Hmmm. What I’ll write here doesn’t exactly answer your question, but maybe it will help.
I just removed an account on a Project Fi phone (running Android 8.1, but the answer above from Adrian is the method I used). Somehow, when I opened the Gmail app, my phone showed email from two Gmail accounts when I chose “All inboxes” from the menu… I wanted to remove one of the accounts so I could use that account with my new RW phone.
Before I went to Settings > Users & accounts > Accounts for (my name), the Fi agent asked how I knew that I had two accounts on the phone (and that I wanted to remove one). He pointed out I might have other accounts stored in an app like YouTube and, from that, think that I had another account on the phone. I also said that the other account’s contacts were appearing in my Contacts list. So we went on.
The Fi agent told me this Settings > Users & accounts > Accounts for (my name) is the only place that accounts stored in the phone itself are listed explicitly. We went there and, sure enough, both accounts were shown. I did as Adrian said (in the link from the previous post here), and the second account disappeared, leaving only one account in the list. So my guess is that, if you have no accounts shown in that list, you don’t have any accounts stored in your phone.
FWIW. Sorry for the guesses. Good luck.