Can I disable plain HTML sign ups?
In the list settings you can find a switch that blocks bots from writing using plain HTML form. If you create a plain form (not necessarily in plain HTML) the bots can sneak in underneath and write in plain HTML anyway.
If you don’t use forms from the editor, we recommend to switch on Disable plain HTML forms signups option in the list settings.
If, on the other hand, you use plain HTML – Disable plain HTML forms signup must be turned off for the form to work properly.