OpenStreetMap now requires API version 0.6. The latest version that supports this by default in JOSM is 1529, which is regarded as stable by upstream. Some but not all versions below this will work with tweaking in the config. Tested 1529 by renaming the ebuild on ~amd64. Given a quick test, but OSM server is currently having post-upgrade difficulties Reproducible: Always
Thanks, we now have 1.566 in tree.