Why are features such as HDMI performance from FreeSync not built into the open source driver?

One of the significant factors is that the HDMI Forum forbids the public from accessing the HDMI specification, as the open source graphics driver would be harmed in exchange.

As the HMDI standard is no longer public, only “HDMI Adopters” are supported by the new open source GPU engine. Even HDMI Forum members including AMD do not, in particular, have open source solutions for the features protected by the closed specification, because certain parts of it can be accessible to the public in turn only indirectly.

In the current case, if users choose to use FreeSync’s open source AMDGPU Linux kernel driver, they can only connect via DisplayPort, but cannot connect via HDMI. This has always been the biggest challenge with FreeSync Linux support.

Many advanced HDMI features, such as HDMI 2.1, which can not be introduced in open source graphics drivers, are threatened by the existing HDMI specification restrictions. The X.Org Foundation is asking the HDMI Forum to allow the publication of at least some HDMI 2.1 specification details so that these features can be (legally) enforced by the open source graphics driver.