Originally posted by The Settler Online:

Dear Steam Settlers,

you have probably already heard the news about the upcoming technology change to a Unity version of our beloved “The Settlers Online”. While we are happy, that you can still settle via Steam at the moment, this will unfortunately not be possible with the new interim Air Client or Unity version afterwards.

Be assured, that we looked into maintaining a playable client for Steam, as we know that many of you really like the platform. In the end we decided to streamline all our Settlers to our official website.
The Steam version will shut down on the 31.12.2020. However, we are able to offer you an TSO account de-partnering from Steam. This will allow you to access your home island with all it's progress via our website.
Please get in contact with our support team as soon as possible, if you want to make use of it.

In line with our terms of sale, we hereby inform you that there will be no refund of any virtual currency you have spent in the game or for any resources you still may own on shutdown date, nor any conversion of any remaining virtual currency to another game.

Thank you for your ongoing support.
See you on the other side 😊

Your Settlers Online Team!