Hey there,
The Exiled lands will still be available for those who choose to stay in them. Most of the additional DLC content included in Siptah will also be available in the Exiled lands, including building sets, armors and weapons so there is value in the expansion for those who do not wish to partake in the adventures found in Siptah. The base game will also receive normal patches and updates like it did before.
Moving to Siptah is not mandatory for those who choose not to.
For more details and questions, please check our stream today at 5pm CEST at twitch.tv/funcom