My unanswered question is hidden/deleted after 15 days. Is it a Launchpad thing, or a thing specific to the individual project? How to find my hidden/deleted question?

I assume that you are talking about https://answers.launchpad.net/elementaryos/+question/403231

Open questions without any action are set to 'expired' after two weeks. The same for questions in the status 'Needs Information'.

To show ALL your questions open any Launchpad page, log in and click on your name in the top right corner of the page or use the link https://launchpad.net/people/+me

On the page that opens click on 'answers', and on the next page select also the status 'Expired' for display and click on the 'Search' button.

Or use this link: https://answers.launchpad.net/~sputnikone/+questions?field.actions.search=Search&field.status=OPEN&field.status=NEEDSINFO&field.status=ANSWERED&field.status=SOLVED&field.status=EXPIRED&field.status=INVALID&field.status-empty-marker=1

Manfred, sure I realized open questions without any action are expired (after two weeks). But what's the logic, what's the rationale? Who benefits from this? I, the the person asking the question find it certainly weird. Not benefiting me at all. Compare it to a traditional internet forum, your thread remains open forever (usually), so if someone find it on Google and has something to add a month or a year later, she can do so.

This question was expired because it remained in the 'Open' state without activity for the last 15 days.