I use two approaches:
- for the simple key plan showing division of partial plans, I export a Revit area plan to CAD, create a generic annotation symbol, add extra filled region, lines, text if needed, and nest it in the titleblock family

- for the key plan that need to provide more than just partial plan division (example below is an egress zones plan),
I create actual Revit floor plan or area plan, scale accordingly, set VG so that it looks clean for that scale, and place it on sheet. If I need to place the same plan on multiple sheet, duplicate as dependent.

- there is another approach that use a legend view as a single legend can be placed multiple times, but I have not come to a need to use this one.