Why do people attribute decisions like that to the competence of the programmers? This is a business decision, nothing else. Most likely, some MBA looked over the numbers, saw a few hundreds or thousands of hours logged for tasks related to supporting Linux, and decided that Proton was good enough. Most likely, no programmer was even asked whether Linux support should be dropped.
And yes, even if you know what you are doing, every build going out to tens of thousands of active players needs to be tested, and that costs time and thus money, which is something every experienced cross platform developer should know.