No exactly true. There are physical requirements for various HDMI versions. The OEM has to validate the port for the version of HDMI they want to support. The driver checks the connector tables in the bios provided by the OEM to determine what connectors are present and what they support.
The Linux driver since some months ago checks those tables, any refuses to utilize HDMI 2.0 features offered by the GPU/APU, even if they work just fine and the manufacturer was just too lazy to "validate" or to update his BIOS image.
Meanwhile the Windows drivers don't give a d*** about that BIOS table and HDMI 2.0 works like advertised for the affected GPUs.
This is clearly (like with ACPI-parsing for so many years) another case of "Linux kernel developers want to do 'the right thing' according to written specification", with just bad results for Linux users because neither Windows drivers nor the firmware follows the written specification.