The mapping for the deprecation logger defines data_stream.type, data_stream.dataset, and data_stream.namespace as constant_keyword fields, with data_stream.type being restricted to being logs. These fields can have different values for different data streams though. Assuming its type is logs, the first entry written doesn't cause a problem because the mapping is not strict and gets updated to have the constant be whatever was inserted. But if the deprecation logger then writes a different value to one of the constant_keyword fields, we get an error like this (seen on an actual cluster in the process of upgrading):

[instance-0000000005] Bulk write of deprecation logs encountered some failures: [[4VK3j5UBamJ0EqvgUJmJ org.elasticsearch.index.mapper.DocumentParsingException: [1:87] failed to parse field [data_stream.dataset] of type [constant_keyword] in document with id '4VK3j5UBamJ0EqvgUJmJ'. Preview of field's value: 'elasticsearch.deprecation']]