In Appian release notes of 21.4. Removal section contains
Process start forms and user input tasks can no longer be configured using in-line expressions. To configure new forms, directly select an interface and map inputs to your process variables. Existing expression-backed forms will continue to work as designed.
What does it really mean?
Discussion posts and replies are publicly visible
Similar to what Gopalk already said - in older versions you could basically hardcode some interface code into a special expression editor box at the Start Form or User Input Task form configuration dialogs - instead of selecting an interface that was created and saved via the interface editor. There was almost nothing useful about this, with the sole exception of being able to make a quick-and-dirty "one button" start form or other things like that, which of course was a hacky workaround anyway.