For example, Errors where encountered while processing and mint is weird - Linux mint [on hold] has been put on hold as off-topic. The poster then posted here.
Should such questions be flagged for deletion, or left as is?
I believe that off-topic* questions should in general be deleted. In particular, questions which have been closed with no good answers are dead ends in searches, so removing them is likely to make the site more useful to visitors. If the question has been asked on another site, there is even less reason for it to live on here.
There are a number of tools to help deletion of such questions happen, such that you are unlikely to need to resort to custom-flagging.
Closed questions are deleted automatically when
Therefore it is most often not necessary to take any action to cause deletion of a post closed as off-topic as the post meets the above criteria. The question you linked to will be deleted unless it receives upvotes.
It is also often possible to cause a closed question to be deleted by downvoting it, because doing so zeroes the score. Its being off-topic is usually sufficient reason for me to downvote a question, though I might not do that if it is very good, or if it is not bad and downvoting will not cause it to be deleted anyway. It is also occasionally possible to cause deletion of a question by downvoting an answer to it, but I do not feel right about doing that unless the answer is actually bad.
Users with 10k reputation may cast delete votes on any question that is closed (and has been so for 48 hours)***, so you may be able to bring about deletion of a post by bringing it to the attention of other users with >10k. Please feel very welcome to visit the Downboat aka the Ask Ubuntu janitor's closet for opinions and help with deletion****.
If the question has awesome answers, I suppose we can just tolerate its presence. I don't attempt to delete questions with obviously good answers even when they are off-topic, though I do take action to close them.
Posts that have been closed only because they have been cross-posted and are actually on topic here (slightly different from the case you mentioned, but related), should imho usually not be deleted if they have answers that appear to be possibly useful.
If you have thought about all the above and you still
then you may want to flag it. I think I have only ever done this for locked posts I have come across which can't be voted on.
Essential reading for those interested in moderation, imho:
* Obviously I mean questions which are actually off-topic. Not questions which are wrongly close-voted or closed as off-topic. Those should be left-open or reopened, unless they are otherwise unanswerable within the site scope and format.
** Duplicates are only deleted if they meet the deletion criteria applied to open questions, which are much more generous and mean that, for example, duplicates are never automatically deleted if they have answers, even if the answers are negatively scored. The only difference in delete-ability between open and closed-as-duplicate questions is that delete votes may be cast on the latter by users with 10k reputation.
*** Users with 20k reputation may vote to delete questions with a score of less than -2 immediately. They may also vote to delete answers with a negative score. Deleting an answer is another method which may bring about deletion of a question in some cases.
**** You can also ping me there and, as well as asking me to help delete things, ask me whether any particular question will be automatically deleted...