Switched from a company using Jenkins to one using GitLab CI, and while GitLab CI is obviously "better" in the sense that it has less historical baggage, there are actually quite a few things I'm missing. Jenkins has a plugin for pretty much every obscure thing you can imagine, which is a blessing for the user and a curse for the administrator, as Jenkins quickly becomes Frankenstein's monster. But every time I have to wade through tons of log ouput on GitLab I miss Jenkins' warnings plugin, every time no runner is picking up my job I miss the nice runner overview of Jenkins which quickly showed you what runners are actually busy with, and every time that old slow runner is grabbing all the jobs I miss the runner prioritization... I could go on here, but really, there's a lot of things that Jenkins could do through nifty plugins that GitLab CI cannot do yet. I even wrote one plugin myself for supporting our in-house Linter, really wasn't that difficult and you could hook into pretty much every little detail (which, again, can also be a curse because every plugin had the power to simply crash your Jenkins...).
EDIT: So to be clear, I'm not saying "Jenkins is better than GitLab". I would say GitLab CI is better designed, more robust and stable, but Jenkins is more configurable, extendable and has more features through it's plugin ecosystem. So personally, I wouldn't go back to Jenkins, but I also don't find it ridiculous that people still use it.