I recall such an option was announced at Mendix World 2019, but I can't really find anything with a few Google searches. Perhaps you could contact your Customer Success Manager or submit a ticket in the support portal to request more information.
[edit]: Johan talks about this feature in this video, starting around 2:30.
Yep Mendix for private cloud is now available:
https://www.mendix.com/evaluation-guide/app-capabilities/mendix-for-private-cloud