The doc said "The revision ID of ... the right revision for diff views.".
But when you enable preferences "Do not show page content below diffs" or use url diffonly=1, this variable return 0.
Compare following two links:
- https://en.wikipedia.org/w/index.php?diff=911547240
- https://en.wikipedia.org/w/index.php?diff=911547240&diffonly=1
Expect run mw.config.get('wgRevisionId') in browser console with second url return 911547240 instead of 0.