Yes it is under maintenance.
Official documentation Salesforce Optimizer, Lightning Experience Readiness Check, Lightning Experience Configuration Converter, and Einstein Readiness Assessor application update says:
The Salesforce Optimizer, Lightning Experience Readiness Check (including Visualforce Check and the Lightning Experience Change Management Hub), Lightning Experience Configuration Converter, Lightning Experience Welcome Mat, and Einstein Readiness Assessor applications have been placed into maintenance mode for repairs.
When trying to access any of these applications, you may see an "Offline for maintenance" message. When an application is in maintenance mode, access to it is disabled, which means that customers are unable to run and use it.
Salesforce does not have any ETA for it, maybe due to holidays. Suggest you to try it at some later time.