markewaite && basil thx for answers
I know it is hard to remove old code because of compatibility with others (plugins, customer shared libs ...)
We have always same issue in our product, like everybody
. I know, this will be long doing process, but there are really benefits, when you has "clean" code base.
Thx also for you link. It was helpful, because I found this tool https://github.com/jenkins-infra/usage-in-plugins . 
This is exact what I need https://assets.ci.jenkins.io/static-files/B8SRSImn48Np_Y5K82RWOr_o9nCJJZvAHlbTOY6q-dkxNjU0NzU3MzEzOTE3Ojk6YW5vbnltb3VzOmpvYi9JbmZyYS9qb2IvdXNhZ2UtaW4tcGx1Z2lucy9qb2IvbWFzdGVyL2xhc3RTdWNjZXNzZnVsQnVpbGQvYXJ0aWZhY3Q=/output/deprecated-and-unused.html
When you agree I will:
+ adapt the tool usage-in-plugins to be possible
+ shown "long" is the code deprecated.
+ send mail notification to plugin maintainers about possible mismatch (when is running in jenkins-master and it is "new-used-deprecation" ....)
+ mark build as failed, when "new-used-deprecation" are found. So it will be necessary to clarify new deprecated code with others (potential dangerous, because some maintainers does not really answer, so it will be hard to mark so one code as deprecated)
+ shall be possible start it for groovy code, that the jenkins-administartos can check own shared-libraries
+ add some logger-severe message in the deprecate-unused parts like:
'This function _FUNCTION_ is deprecated and will be removed in future. See also https://issues.jenkins.io/browse/JENKINS-68664 for more information's'
+ add some logger-warning message in the deprecate-used parts like:
'This function _FUNCTION_ is deprecated but can not be removed, because is still in use. Please inform maintainers about this. See also https://issues.jenkins.io/browse/JENKINS-68664 for more information's'
+ and / or add admin-monitor which can inform jenkins-administrators about that.
So everybody can send us (post it in this ticket, or somewhere else (maybe new JEP will be better) the list of functions and we can merge it. Then can be "safely" removed after a bit time period (Ex: 3 months)
The steps described here, might be used in future, when some body marked the code as deprecated
ps: I think this is "similar issue" https://issues.jenkins.io/browse/JENKINS-31035 therefore it can be linked.
ps2: Pfuh. I see this will take a while and need much effort from my side (again) 
As far as I've seen, we intentionally do not remove deprecated methods from Jenkins API's. We retain compatibility by retaining deprecated APIs because that is a good way to retain users.
There have been exceptions to that general behavior. However, they are usually preceded by a detailed analysis to confirm that Jenkins plugins have been reviewed for incompatibilities, pull requests have been submitted, and sufficient time has elapsed that the maintainer could have merged and released the pull request.
This is a topic that is better suited to a developer mailing list discussion before you embark on the effort to remove deprecated APIs.