Definitely true, and worth responding to. (For instance, with a specific Internet Archive source or a link to the Wikipedia version history instead of main page

Definitely true, and worth responding to. (For instance, with a specific Internet Archive source or a link to the Wikipedia version history instead of main page.) But it's also uncomfortably true that academic works have an ugly tendency to disappear post-citation. Less so in mathematics, I'm sure, but even in the hard sciences far too many papers out there reference things that weren't digitized, rely on data that wasn't published with the paper, or otherwise hinge on something you can't actually get to. There's probably no great answer here, it just feels like authors and editors sometimes use "it was reputably published once" to shrug off disappearing sources.