Improve deprecation process and provide BC so pipelines do not break
The current "deprecation warning" job in the .pre only detects deprecated variables if they have been defined at the job level variables: section. For variables defined within jobs, such as composer (next major), the .pre job does not know about them and therefore is not added to the pipeline. Then the 'composer (next major)' job just fails without any warning, and with no explanation that it is caused by a deprecated variable.
There are several ways this deprecation process can be improved, to give the messages in more than one place.
Edited by drupalbot