Work for this feature is now planned. Will update here when work on this feature starts and when it becomes generally available.
Thank you.Ruslan commented
Not everything is about RU usage, this is simply bad dev experience. It's not super straightforward, when you decide you need to order by a certain field, to process all existing documents in the database so that they have the required lower-cased property. We have the ability to do that in our project thanks to our custom schema versioning solution, but without it it's not an easy task in a big database, and it's a lot easier for devs to give up on "nice to have" features.
I suspect it's difficult to implement, considering the number of optimizations in place in Azure Cosmos DB, but please revisit this soon.