There has been a message on the Steam TSO Community web page - part of which reads:
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.
Nothing about is not working at the moment.
This was writen on page 8.