This retirement notice is just for the Creator platform. Azure Maps as a whole is not going anywhere and is growing quickly. I believe creator is a fairly expensive service that is used by a small number of customers. Personally, I always felt that service was way over architected for this scenario.
As for a migration path, I don't believe an official one has been documented yet, but is one way is to convert your building data to geojson and use load that directly into the map. Use the underlying styling layers in Azure Maps to filter by level/oridinal, and color your data as desired.
As for indoor maps, I would consider converting the data into the Indoor Mapping Data Format (IMDF) which is a new open data standard for indoor maps. https://docs.ogc.org/cs/20-094/index.html IMDF and the underlying ontology of Azure Maps Creator are pretty close. Also, with IMDF, you can upload it into Microsoft 365 Places to power a lot of business scenarios: https://learn.microsoft.com/en-us/microsoft-365/places/places-overview Also, if it is a public building, you can also submit it to apple to be included in iOS.
I've been experimenting with IMDF and working on creating a reusable control for integrating with the Azure Maps Web SDK. Hoping to have a first version ready in the next month or two.