Fixing this bug properly requires us, on upgrade, to force a full re-import of all textures, regardless of whether the project is affected by this issue or not.
In the case of the 2021 LTS, we believe that a backport is thus likely to create more pain than it would remove: as such, we are opting to close this port as "Won't Fix".
UI and documentation for 2021 LTS will be updated to reflect the fact that the "Read/Write" toggle has no effect on Texture2DArrays and Texture3Ds.