This is a reference to achievements listed on the Discontinued 1.0 Achievements List, most notably the part how it mentions that some achievements will change on how they work.

I thought I'd check my achievement privilege and ask to make sure there aren't any other achievements that were discontinued behind the scenes.

Just as someone who spent too much time getting all of those that were listed (having to do spearfishing within a week was kinda fun lol), I wouldn't want to be upset learning I could've/should've/would've gotten another 1.0-only achievement.

I would probably punch a pillow.

Think of the pillows.