The bug
Much like how the black "currently selected" box used for resource packs and data packs cannot be retextured (MC-269443), the red rectangle indicating an incompatible pack cannot be retextured either, and appears to be hardcoded,
How to fix
Introduce a texture file for this effect (an opaque red rectangle with appropriate scaling mcmeta should suffice).
Linked issues
relates to 28
MC-278413
Chat background cannot be retextured
Open
MC-264571
Both standing and wall sign GUI backgrounds cannot be retextured independantly from the block
Resolved
MC-268805
panorama_overlay.png no longer works
Resolved
MC-268927
The dark background in inventory screens cannot be changed via resource packs
Resolved
MC-269079
Death screen background cannot be retextured
Resolved
23 more links
As far as I can make out this is not present in the assets folder of the game .jar