Intel will halt UEFI bios compatibility class 0, 1 and 2 support in 2020. From that moment onwards only UEFI class 3 will be supported. This means that Intel platforms cannot boot through the Compatibility Support Module mode, and that's a problem for dual OS boot systems.

Intel this way pretty much halts the traditional legacy BIOS support. The CSM provides additional functionality to UEFI. This additional functionality permits the loading of a traditional OS or the use of a traditional OpROM The news reached the webs though Brian Richardson, a developer at Intel. Recently he held a presentation on the topic at the UEFI Plugfest in Taiwan, reports tweakers today.

According to Brian the compatibility mode is still here as people want to bypass say the secure boot method, or have multi-OS boot settings. An advantage to discontinue the CSM support is that it will make room in the firmware for other stuff, initially, the BIOS would get significantly smaller in file-size. Also security-wise, the step would make a lot of sense.

Starting 2020 Uefo Class 3 will be the new standard at Intel, with secure boot enabled.  

Intel Halts Certain UEFI BIOS Class Level 2 Compatibility Modes In 2020