The SDK Release and Maintenance Policy has been updated this year.
Series 0 & 1 devices will continue to be supported through the Gecko SDK.
Series 2 & 3 devices will now be supported via the Simplicity SDK.
The Gecko SDK will include a Long Term Support (LTS) version, while previous versions are being archived as outlined in the official documentation.
For more details, please refer to the policy page: Silicon Labs SDK Release and Maintenance Policy.