Advances in the java programming language, version 16 and newer, slashed a million lines of code from my codebase. Maintaining my programs became easier overnight, due to this 1 secret trick: Records. Unfortunately version 16 was not LTS, so I had to wait until this year’s release of version 21, which is LTS. Go read the linked article. It explains Java Records in a very approachable manner.
Groovy did this ages ago- I don’t do much groovy anymore, but the more I use other languages (nodeJS, python, etc) the more I appreciate all the developer friendly language features it has (and has had for over a decade). I find myself having to use lodash and its variants on other platforms to make up for the lack of all the really useful functional methods and lambdas.