As of about 23:00 UTC on April 26, 2018, Ubuntu Bionic Beaver 18.04 LTS has been officially released. The corresponding release announcement email that went out to the ubuntu-announce mailing list is here.
Therefore, effective as of this post, new 18.04 questions are now to be considered on topic here so long as they comply with the other requirements for on topic posts as defined in the Help Center and here on Meta.
This does not mean that 'older' 18.04 questions can be reopened without discussion or community consensus. Questions which were closed because 18.04 at the time was off topic or questions which are related to upgrading or using 18.04 prior to its official release remain offtopic, unless the community at-large believes (on a case-by-case basis) specific posts should be reopened.
CLOSE REVIEWERS: Do not close new 18.04 questions as "offtopic", effective as of the time of this post, unless they are offtopic for some reason other than the question being about 18.04