IMHO such questions can be reopened if they are about something that is reproducible in the released version. I voted to reopen the question you linked to. The fact that you got upvotes for your answer subsequent to the release suggests that the material remains useful so should probably be maintained.
Note that closed as off-topic questions with no upvotes and no upvoted or accepted answers will (thankfully) be automatically deleted by the system if they have not been recently edited. This keeps the site clear of a lot of useless things that would otherwise lie "dormant" as you mentioned. However, since a single upvote or upvoted answer will preserve any question forever even if it's closed, it is a good idea to try to clean up any closed beta-version posts that remain after release, either deleting or reopening them. A few users usually take responsibility for reviewing posts closed as off-topic for this reason around the time of a release. Usually, most of them relate to transient bugs and are dispensable, but a few are reopened. See this meta post for an example of such activity.
Here is a poorly written SEDE query I made to find questions about 18.04 posted in the first 4 months of 2018 which are closed as off topic. It is imperfect because it only searches the post body for the string 18.04, and the PostHistory table does not provide the ability to distinguish between different off-topic reasons, only between the main reasons. Still, it probably finds most of what we're talking about.
I will try to find some time to look through the 43 posts found by this query to see which should be deleted or reopened.
