Is matter actually an open standard? When I last looked into the CSA stuff you had to pay a few thousand dollars a year to become a member to access any specifications
Disclaimer: not speaking for my employer or CSA/Matter (I have done paid work on the reference implementation).
Don't know if it's what you're looking for but downloadable pdf docs for (Zigbee) Rf4ce specifications are at the CSA Resources page:
https://csa-iot.org/resources/developer-resources/
EDIT reply to: >Matter is not ZigBee.
Setting aside the radio incompatibilities, various web articles says Matter was Zigbee Alliance "Project CHIP" and later rebranded to "Matter". And "CSA Connectivity Standards Alliance" is itself a rebranding of "Zigbee Alliance".
If the pdf reference docs are not relevant, I guess I don't understand why they're hosted at the domain "csa-iot.org".
Thread uses the IEEE 802.15.4 PHY and MAC.
Zigbee also uses the IEEE 802.15.4 PHY and MAC, using the same frequency bands as Thread as far as I understand it.
Maybe a new radio is needed if you want BLE commissioning (is this required?), but for operation any chip with a IEEE 802.15.4 PHY should support both Thread and Zigbee.