That seems rather sloppy to start with: you could easily end up a year or two out (the date that matters isn't the date of publication, it's the date that was in the author's head when they wrote something like "Twenty years ago").
In any case nowadays I think there's no excuse for this practice continuing. It's far too common for a paper to get sent around, or put on the author's personal website, with no associated date at all.
Yes. It's become a real problem. Sometimes you have to look at the references, which do have dates, to get a rough idea of when the paper was published. Or at least what date it could not have been published before.
> you could easily end up a year or two out (the date that matters isn't the date of publication, it's the date that was in the author's head when they wrote something like "Twenty years ago")
Doesn't matter. If the author was thinking of a specific year, they'd say the year. If they say "20 years ago", everything that was "20 years ago" this year is also "20 years ago" a couple years from now.
It's common for papers to include words like "recently", or indeed "has not yet", which are rather more time-sensitive and still don't specify an exact year.
In any case nowadays I think there's no excuse for this practice continuing. It's far too common for a paper to get sent around, or put on the author's personal website, with no associated date at all.