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?
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Whilst on the topic, you're able to provide "inline expressions" in various places in Appian (Scrip Tasks, Record Action Visibility etc etc). But it's bets practice to encapsulate anything bigger than a trivial piece of code as it allows you to maintain and test it independently of the object it's used in and often it leads to reusable components, which again reduces the maintenance burden.