The historic "layer" is actually just the same layer as your active parcels. When you pull a Parcel Fabric into ArcGIS Pro, it automatically creates two entries in the table of contents per parcel layer, one for active, one for historic.
The way Pro keeps these features straight is using definition queries. If you look at the query on your active parcels, you'll see:
retiredbyrecord IS NULL
Meaning that any parcel without a value in the retiredbyrecord GUID field will appear. When you retire a parcel in the parcel fabric, the GlobalID of the active record is inserted into that field.
Similarly, historic parcels have the query:
retiredbyrecord IS NOT NULL
Meaning that any parcel with a value in that field will appear there.
Again, your active and historic parcels are all in a single layer, and it's the auto-applied definition query that is making them appear separate.
- Josh Carlson
Kendall County GIS