The original version of a post edited during moderation is not stored anywhere, apart from in email notifications. Canapin checked the logs and database tables, including posts, revisions, and reviewables, and found no record of the original content. However, anon65426961 notes that within the default 5-minute edit grace period, no record is kept of the original post if edits are made within the character limit, which can be problematic. To avoid this risk, anon65426961 set the edit grace period to zero on their forum.
Continuing the discussion from User's unedited post sent by email even when post edited by admin after approval:
When a user’s post is held for moderation, edited, then posted, is the original, unedited post stored anywhere? (Apart from in all the email notifications!) Thanks.
Hi Jonathan,
I looked at the logs (all staff logs) and database (tables posts, revisions, reviewables…) and it seems there’s no record of the original content.
Only a record of the reviewable item is kept.
If you think this could be useful to get access to this, with use cases, I invite you to create a new Contribute > Feature topic 
With the default of five minute grace for edits of new posts, my understanding is there is no record kept of the original post if any edits are made within those five minutes as long as they don’t surpass the character limit of edits for edit history to be published.
That can be problematic for a lot of reasons so I set the grace to zero for my forum to avoid risk of having no record of possibly important edits.