Set reference scale on layer independent of map's reference scale
Right now the only recourse for getting a heat map layer to appear the same regardless of extent and scale is by setting a reference scale on the map. This presents problems, though, when adding an inset of the same map to a layout. If the inset uses a different scale, the appearance of the heat map layer comes out as desired -- sure -- but then everything else in your map from labels to scale-based symbology is suddenly comparatively tiny, which is undesirable. If the heat map layer could keep its own reference scale, this problem would go away, and I can imagine there are other circumstances where setting a reference scale on just one layer would be helpful.
The workaround for now is to go through every single layer's properties (can't multi-select or modify at group level) and uncheck this:
The heat map use case is addressed in ArcGIS Pro 3.2.
As pointed out in the comments, the more generic request exists here https://community.esri.com/t5/arcgis-pro-ideas/reference-scale-for-single-layers/idi-p/926683/highli...
See the What's New documentation for more new features in Pro 3.2.
The Your Ideas in ArcGIS Pro 3.2 blog and video have been posted to the Esri Community blog. You can take a look at these to see your idea and all others that were implemented in this release of ArcGIS Pro.