I was informed by Collabora that Timothy Arceri left Collabora at the end of January to join Valve full-time to focus on the AMD Linux drivers. Arceri had been working for Collabora after he began working on Mesa. Timothy started contributing to Mesa via crowd-funding work on different extensions in 2013 and he was successful at doing that while learning Mesa and then landing the job at Collabora and now Valve.
Timothy's initial work now at Valve appears to be focused on Radeon on-disk shader cache support.
Timothy follows Samuel Pitoiset who joined Valve to work on the open-source AMD Linux driver stack last year. Valve also has former AMD developer Andres Rodriguez working on open-source AMD Linux too. This all materialized after Valve mentioned at Steam Dev Days they were looking to hire folks to work on the open-source AMD stack and Valve wanting to improve the open-source AMD driver for VR. Now it appears they have at least three full-time developers working on advancing the open-source AMD Linux driver stack including RadeonSI, RADV, and the AMDGPU kernel driver.