As you know, some questions are automatically deleted. Indeed, this also happened in this case.

Specifically it seems to fall under 10 in that thread How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?

10. The system will automatically delete unlocked, unanswered questions older than 365 days on main (non-meta) sites, that have a score of zero (or have a score of 1 and the owner's account is deleted), have fewer than 1.5 views per day on average, and fewer than two comments. (RemoveAbandonedQuestions)

Note that it was deleted just a little over a year past after it was posted; this script does not run daily, as far as I know, so some shift is expected.

That said, I agree it is rather surprising to have a recently edited question with a bounty deleted in this way. Possibly this is worth bringing up as an improvement; it may even be around somewhere.

As you know, some questions are automatically deleted. Indeed, this also happened in this case.

Specifically it seems to fall under 10 in that thread How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?

10. The system will automatically delete unlocked, unanswered questions older than 365 days on main (non-meta) sites, that have a score of zero (or have a score of 1 and the owner's account is deleted), have fewer than 1.5 views per day on average, and fewer than two comments. (RemoveAbandonedQuestions)

Note that it was deleted just a little over a year past after it was posted; this script does not run daily, as far as I know, so some shift is expected.

That said, I agree it is rather surprising to have a recently edited question with a bounty deleted in this way.

Indeed, it might be a known bug, according to Question with an active bounty was mistakenly deleted by the Roomba which mentions Prevent questions with bounty from being auto-deleted that is an implemented feature request.