The get field mapping api currently checks for data read blocks rather than metadata blocks. Seems like a bug given that the api allows to retrieve mappings and not data. This can be seen by adding a metadata block to an index and then calling the get field mapping api, which works while it should be rejected.

The problem might stem from the fact that metadata block currently means read and write, causing different issues, which will be solved with #9203. Once that is in we will be able to only check for metadata read blocks instead.