The IN_MEMORY workspace was and is a great addition to ArcMap, boosting performance and helping with managing temporary data. I use it regularly.
ArcPro offers IN_MEMORY but has the strange behaviour of moving it to a file geodatabase if the data is being displayed in ArcPro. This is documented here.
My idea is to resolve this strange behaviour so that anything written to in_memory stays in_memory and not written out to the default geodatabase. Writing the data to a geodatabase is very frustrating as it defeats the whole purpose of it being temporary. Often I'll name it something quick like zzz as I am testing some logic so you end up with all these nonsense datasets filling up your database. This brings an overhead to cleaning up the geodatabase and ensuring it's optimal with compacting. A genuine step back in usability.
None of this is an issue in ArcMap, in_memory stays in_memory and closing the application makes it all simply go away. In ArcPro I think I am writing to in_memory, close the application, re-open and then discover all my temporary datasets clogging up my database...
This has been marked by Kory Kramer as a Duplicate of Save Feature Classes to in_memory workspace in ArcGIS Pro Map