In 5.5 we no longer show field conflicts for "like" types and allow you to select a field for a visualization that resolve to the same Kibana type. This is best described in this comment #13070 (comment)

Today, Kibana will not allow you to select a field for a visualization if there is any kind of conflict. While the adjustments made in 5.5 makes this experience a bit better, there are still scenarios in which field conflicts occur for dissimilar types. For example a field who had text type and the mapping changed to integer.

We should provide an outlet for users to override the assumptions Kibana makes about field types. If there is a conflict, a user should be able to toggle an advanced setting to ignore these conflicts and allow a user to select this field regardless. If the visualization attempted to aggregate on a non-supported or conflicted type, Elasticsearch should throw an error and surface this in Kibana.