Modules of deprecated functions automatically get deactivated during app installation.

Read in the function's documentation how to replace the deprecated function.

If you still want to use the deprecated function, you can activate the corresponding module manually in your "manage apps" section of Jira.
Here is an example for the Number Compare Validator:

Overview of deprecated functions

function	replaced by	enabled until	enableable until	note
Number Compare Validator	Field Compare Validator		30.06.2020
Number Compare Condition	Field Value Condition		30.06.2020
Copy Field Value from Parent to Sub-Task Post Function	Copy field value(s) between related Issues Post Function	01.012.2020		Only existing configurations can be changed. New configurations can not be added.
Copy Field Value from Sub-Task to Parent Post Function	Copy field value(s) between related Issues Post Function	01.12.2020		Only existing configurations can be changed. New configurations can not be added.

Browser not supported