Did you notice that there is no more “Open Checklist” in the File menu drop-down of X-Plane12? I didn’t even notice when it was actually removed. True, it was a little limited, but for those who want to keep their installations “plugin free” that option just isn’t there anymore.
However, having stated the bad news, all is not lost. I presume one of the reasons for the removal is that there is a very good – very stable – absolutely no trouble – freeware – plugin available called XChecklist created by @sparker . If you have never seen this plugin, download it and try it out. There are many, many checklists published on this forum which are compatible with X-Checklist, so you are are bound to find one for your favourite aircraft. In fact, it is very easy to create your own checklist with nothing more than a humble text editor.
The plugin will work on Windows, Mac and Linux. And the good news is that it is far more capable than the original Laminar checklist.