If you pad your listings, people will assume you need to because there _aren't_ many such jobs. Moreso over time as the ratio of filled listings to active recruitment grows -- if all visitors ever see are jobs that no longer accept applications, they will not come away with a positive outlook on the field.
It's like arguing that a news site should delete yesterday's news instead of keeping an archive, because "you're only interested in today".
As for your "sustainability" and "career building", I disagree there too, but that's for a different conversation.
As someone who recently was job hunting and come across another site with this problem: absolutely do this. You get the bonus of instantly showing the user which jobs are current, so they don't notice only after wasting time and then jump to the assumption the site is dead. You show users that you take the up-to-dateness of the data seriously. And lastly you get to keep the archive around, which I agree with you is a good thing to do.
> There's plenty of recent jobs listed, so your point about "only ever seeing jobs that no longer accept applications" is moot.
Not for me there's not. I'm in the UK and it's all 2 years old.
(Good idea tho, hope it continues well!)