People working with Jenkins pipelines can have very varied backgrounds. They can not be assumed to be developers, they might barely know about Groovy or object oriented programming.
To tame this complexity the Declarative Pipelines were introduced, which are somewhat simpler than the regular Scripted Pipelines.
However some parts of the configuration are still quite hard to follow. One example is the email-ext-plugin configuration.
I'm not entirely sure how this could be improved, but I believe a first step would be to get rid of these kinds of $class configuration entries:
Quoting Andrew Bayer: