This article no longer seems to be receiving updates, here is some more recent information to close the loop. (I would suggest any future comments to be posted in the active discussion in the article linked below:
1. As Chad mentioned above additional details like supported features via the gateway services and lengthy discussions on unsupported features like BToE, Boss/Admin, and others can be found in this newer article:
https://techcommunity.microsoft.com/t5/Skype-for-Business-Blog/Skype-for-Business-Certified-IP-Phones-with-Microsoft-Teams/ba-p/286073
2. No new functionality or features will be added to the existing gateway services, as stated by Microsoft in the discussion linked above.
"3PIP phones will continue to work with Teams for "the foreseeable future" but only with the functionality above. They will not support any new Teams capabilities, ever."
3. In the light of the recent announcement that Microsoft will be retiring Skype for Business Online on July 31, 2021 the gateway services providing basic Team functionality to 3PIP-qualified phones will continue to operate and be supported by Microsoft through July 31, 2023.
https://docs.microsoft.com/en-us/microsoftteams/faq-journey#device-compatibility
4. Microsoft just posted this article today which also summarizes the latest messaging:
https://techcommunity.microsoft.com/t5/Microsoft-Teams-Blog/Skype-for-Business-phones-3PIP-support-with-Microsoft-Teams/ba-p/789351