Web service plaque is like that stuff on your teeth. In a way, it's like that sticky yellow paste that forms when saliva and food and fluids form that you need to brush off at least once daily.
Sign-up forms that disable pasting or autofilling in the form.
I create unique passwords for every site, and sometimes need to paste in a password (that 1Password won't fill). You can (nay, should) enable this on your website: https://developer.apple.com/documentation/security/password_autofill/enabling_password_autofill_on_an_html_input_element
Two-step login forms, the first one requiring your username or email address, then going on to the password form.
This one is at least understandable, since it looks to see if you have single sign-on, and redirects you to that service if you do, and asks you for a password if you don't. If it doesn't redirect to you a single sign-on service, though, then this is web service plaque.
Sign-up forms that make you type in your email address twice.
I have my email address in a text expansion and in an auto-filling software (all operating services do this now, and software like 1Password can autofill every form, potentially).
I've felt this way since 2008!