Jenkins only stores build histories for ~30 days, so any history past that has been deleted AFAIK. We need to backfill this data to get a good idea of what the historical trend is and was for test code coverage data.
I very naively tried to run coverage in a custom docker container (https://gitlab.com/legoktm/tool-coverage/tree/master/docker) however the results I got were one percent off (a significant difference) what I should have been getting, probably due to environment differences (jessie vs stretch, php5 vs php7, etc.).
I think it's better if we just figure out how to run the Jenkins code coverage job, but against historical versions of MediaWiki core. I believe this should be possible via the parameterized builds feature. I'm envisioning a script that determines which core commit to run against, triggers a jenkins build against that commit, waits until it finishes, grabs the clover.xml file, and repeat going back further in history until it stops working.