Wikimedia Bugzilla is closed!

Hiding the legend by default makes it easy to miss entirely, and in many cases it is likely to be needed - interwikis do not often need changing, so the user cannot necessarily be expected to remember what each column does when they do. (I not only forgot what the columns did, I also forgot there even had been a legend.)

Given that the legend is relatively short and pretty easy to ignore when not needed, hiding it by default seems somewhat unwise. Though the explanations could be better, they are still better than nothing especially when the user is messing with the database in such a manner as this extension enables.