For the user to be able to activate/deactivate and put conditions on the different user and groups mapping criterion we use for automatic mapping during a copy.
I would even go further and treat the auto mapping as a bug. The reason for that is very simple.
There are many users with similar surname like Müller, Gruber, Murphy, Schmid etc.
As a site collection owner, I really want to avoid that USER B is having permissions now on my content because USER A who already left the company (or was never at my company) had a similar surname, display name, mail address, etc.
Sure, if the account name or display name are the same I would expect this but everything the SharePoint people picker cannot resolve should be treated as unresolved and only be automatically matched if one explicitly allows / configures this.
In some cases, this automatic mapping might be helpful but most of the time this makes absolutely no sense for me.
One might get in real trouble because it looks like USER B has created or modified a file he / she should never have permissions for. Regarding the official documentation it might also be the case that “A group can be matched to a user and a user can be matched to a group”. This means that a lot of users might get access to my content because the automatic mapping found something.
Currently there is no way to get around this except using explicit mapping file. But how should I generate a mapping for users I never had in my Active Directory or left the company long time ago?
Also, there is currently no way to see why ShareGate is using USER B for its mapping. I did not find any entry within the reports.
I would also like to add the ability to pre-check how the user mappings will occur based on auto mapping, and which ones still need to be mapped. hopefully with the option to pre-populate the mapping table.