That law provided neither penalties nor funding for improving ETC compatibility, so it's no wonder most states haven't bothered. California, for instance, already has statewide interop – and has for decades. In fact you can use your transponder for parking at SFO. I believe the various toll agencies out here have been issuing "6C" compatible transponders since 2019 with the goal of completely phasing out the old "Title 21" transponders by 2024. ISO/IEC 18000-63 (6C) is being used by Washington, Colorado, Utah, Kentucky, Indiana, Georgia, North Carolina, and Louisiana.

For someone doing a lot of interstate driving NationalPass already offers transponders that are compatible with a bunch of different systems (E-ZPass, TxTag, Title 21, and EZ-TAG), supposedly also including 6C so there's that.