this fault is not only from the devs, GOG got part of the fault
the decision about discontinuing GOG version support was taken by devs, not the platform. The problem is that even though GOG could notify users on their own - GOG is not the authority who can offer any options except refunds. At the same time, devs as main decision makers could ask GOG to send notifications for all of the customrers offering to transfer licence to steam or any other supported service. But currently they did nothing, and as I understood from their silence they are not planning to bother about GOG customers at all. Just because they have their money already and no actual obligations. Only if you contact hem directly - someone probably will spend a minute to make you a great offer of steam key.
As I understood from previous topics, devs were not going to support GOG version since previous beta release, around half a year ago. So it's not a 'fresh' decision. And still there was no notification at all.