The companies in question are going to get in serious legal trouble if they fail to comply in verifying the ages of their users. Microsoft, Google, and Apple are all going to give these companies an API that will move that burden from themselves to the makers of the operating system. Discord, to use an example, will be able to say that they asked Microsoft, Google, and Apple to verify ages, and then that they relied on the information that they were given. What's more these companies actually want to know the age of their users. This information is valuable, and they will be able to use it to target their customers with advertising.
What's more, every bad actor is going to try and target this exception. Linux traffic will end up in the same bucket as bots, predators, and any other malicious traffic. There is nothing in the current exception that requires companies to work with Free Software. My guess is that if your operating system can't do age verification that is blessed by Microsoft, Google, or Apple that most sites will simply block your traffic.
This is going to end up being a huge step backwards for Free Software clients.