I see that version is not going to be migrated. I can appreciate it may not have be especially useful for MediaWiki bug reports that are 10 yeas old.
For pywikibot, which is being migrated for free, we have two different git repositories: compat (1.0) and core (2.0), and the version field in Bugzilla represents one repository or the other. I and others have spent a lot of time (think many hundreds of manhours) sorting through old bugs imported from sourceforge.net to determine whether they are v1 or v2 bugs. Or if the version is unspecified, it means someone hasnt yet determined which version the bug report applied to.
IMO the version field needs to be migrated to a proper field in Phabricator; not dumped in the description field of pywikibot bugs. It would be heartbreaking to see all bugs from the two versions merged back together. It is the common case to filter bug searches by version=v1 or version=v2, as two codebases are very different now, and even backporting bugfixes from v2 into v1 would adversely affect its stability. But v1 bugs are very useful for the many continuing users of pywikibot v1, if only to learn that a problem encountered is already known, who else has encountered it, but also there are patches, workaround, etc available in old bug reports.