Deleting field revisions directly from the database With full recognition that this is not considered a best practice (and is generally considered a worst practice) does anyone know if it is safe to delete field revisions directly from the DB via a DELETE query assuming I can guarantee that only the latest revision is being used/referenced everywhere on the site. 

I ask this question because I have a particular field that has grown so large the our update hook is taking too long. I'm looking to cheat and seeing if anyone has had experience with this.
There was a module in 7 that tried to solve this, Field SQL No Revisions. I am not sure how far along a Drupal 8 port is, but this is where you might want to start. However, that module keeps those tables empty, instead of just the 'latest' record.

Previously (7.x), I used to have hook_cron routines that would purge revision tables of any old data not being used to keep size down. We had some DBs that were huge due to that, which made backups and dev syncing take forever and be cumbersome. It really helped. You'll want to be careful and use the respective API over a database query, so the rest of Drupal is informed about what is happening and can take action (or if your site has no dependencies it might not matter, but it is always, always safer).