We have created a relatively large draw.io diagram, and during its creation, the author has saved almost a thousand times (all very minor changes).
The revision history is now huge - is there a way to delete individual revision history entries for draw.io diagrams (similar to deleting revision histories for Confluence pages)?
It depends if you want to keep only the latest revision or just remove some historical revisions.
Keep just the latest revision
Make a copy of your diagram by clicking 'Duplicate' on the macro toolbar in page editor.
That would create a copy from the latest revision. This would keep the original diagram just in case.
Remove some revisions
You can remove attachment revisions from the attachments view. You would need to do it for two attachments, XML that stores the diagram contents and the PNG. There could be some 3rd party plugins that can automate this as it can be tedious if you are removing hundreds of revisions, times two.
Keep in mind that drawio macro has a revision parameter which makes it display a particular diagram revision. Browsing through a page history will make it unable to display certain diagram revisions.
In either case, it's best to experiment on a copy of the page, just to be safe.