As a result of T134675 which converted Special:MergeHistory to OOUI, the functionality of assuming that if you tick no particular box you want all revisions merged has been lost. Basically, if you wanted to merge all available revisions you could simply hit "merge revisions" – now you have to tick the most revision before you can get anything to happen. The default assumption should be that if you hit "merge revisions" you want all the revisions unless you specifically tick otherwise (similar to how it used to be and how undeletion works).
|mediawiki/core : master||Special:MergeHistory: Merge all revisions if no particular box/button is ticked|
Here's an example. This is what immediately appears after clicking "show mergeable edits". If I click "merge revisions" now (even in this very basic example where there is only one possible revision that could be merged), it will not do anything. I have to first tick the box next to the revision in question, which strikes me as the opposite of what the default should be.