Hi John.
These tables were removed as part of the job assignments feature. In the changelog, it's TL-8945, but if you want to check the patch then it is TL-2082 (I'm not sure how we mixed that up). It's actually named wrong in the changelog - look for a mention of the "position_assignment" table. And there's no mention of the history table - it was not in use so was deleted. I'll put in a patch to fix the changelog (will show up in Totara 9.4).
The easiest way to see what happened is to look at totara/core/db/upgradelib.php, lines 417 to 627, titled "Part 2", "Part 3" and "Part 4".
In Part 2, pos_assignment 'aspirational' records are moved into the 'gap_aspirational' table.
In Part 3, pos_assignment primary and secondary records are moved into job_assignment (the pos_assignment table is actually renamed and columns/indexes are added/removed/renamed, so record IDs will be unchanged).
In Part 4, the history table is deleted because it is unused, although we skip deletion if there are any records in it, in case it is being used in a customisation.
Nathan