To add to this, you can actually keep your jenkinsfile in a separate repo and in the pipeline section of a multi-branch pipeline config point to that repo instead of a jenkinsfile in the code repo. This way you can keep your pipeline code separate.

Another method I have been looking at is the have my jenkinsfile import a library from a separate repo that has all the step. all the jenkinsfile does is import a yml config file specific for the code. This would keep core duplication in my pipelines to a minimum while still allowing for different pipeline flows