I have noticed a change of behavior in Lightroom 6.2.1. When I use the "Photo Merge - HDR" functionality, Lightroom modifies original image files (in case those are JPEGs or TIFFs) or writes/modifies original files' XMP sidecar files (if original files are "proprietary" RAW files, have not tested any DNGs). This happens in spite of having the "Automatically write changes into XMP" unchecked in the Metadata section of Catalog Settings. Judging from comparison of "before" and "after" XMP files, it seems that only the history record is overwritten.
Additionally, I checked the "Photo Merge - Panorama" tool and it behaves in the same way.
LR version 6.2.1 was the first version where I noticed the problem. I checked the new LR 6.3 this morning and it behaves identically.