It used to be possible, yes this is the “new” look of editing
I found the same. I suggest reporting this as an issue via the support portal.
As an immediate workaround, I found that if you first select ‘Format’ = ‘equal to’, it then allows you to select text fields.
Update for anyone who cares: I reported this as an issue. Apparently it is expected behaviour that you cannot see the option to apply conditional formatting to a text field when you first create a rule. This is because the default format is ‘along a scale’ and you cannot apply this to a text field. So as soon as you select any of the other formats you will have the option to select text fields.