I am looking into archiving site collections based on use and the web property LastItemUserModifiedDate seems to be perfect. However it seems like the property is not available in search and iterating the 300.000 site collections might be a slow proces. I wonder if the managed property Lastmodifiedtimeforretention is just another alias for LastItemUserModifiedDate, but I can't find any documentation about that property. Any ideas how to locate the site collections without any user updates within the last x days?
I have pulled some data from a production tenant and it looks like Lastmodifiedtimeforretention in most cases is the same as Created date :-( AND it looks like LastItemModified isn't a good indicator either.