This change was added in Unity 2021.2 (see https://unity.com/releases/editor/whats-new/2021.2.0: "Restricted DXT/BCn texture compression to textures with multiple-of-four width and height. This ensures the same behavior as the Texture Importer and requires multiple-of-four dimensions when compressing."). However, this will not be backported to 2020.3 since it is a change that may potentially break existing (and fully functional) user projects. If there are particular bugs with non-multiple-of-four BC-compressed textures in Unity 2020.3 (for particular compression formats), these should be handled separately by fixing them instead of by adding restrictions.
Note: in Unity 2020.3, DXT-textures with non-multiple-of-four sizes are stored in compressed form, but uploaded to GPU in uncompressed form. This behind-your-back-decompression has been dropped in Unity 2021.2 in favor of simply not allowing this compression to begin with.