Work has continued on improving the address output of results. Administrative boundaries are no longer assigned a fixed type but their context is taken into account. This solves many issues we had with the urban/rural divide. addr:* tags are now fully taken into account when searching and when displaying the results of your search. Especially rural US will benefit from that.
The HTML output has been removed from the API. We have created a separate project nominatim-ui which implements the former debugging interface as a simple javascript application. The documentation has hints on how to migrate to nominatim-ui should you need the HTML output on your own installation.
A full list of changes can be found as usual in the Changelog.md.
If you want to update from 3.x.x, there is a database migration path described in the migration guide. However note that migration to 3.6.0 is rather extensive and you can only benefit from the improved addressing after a full reimport. Migrating from 3.1.0 can be done simply by following the migration guide nonetheless. When updating from 3.0.x, a full database reimport is strongly recommended. Updating from older releases is only possible with a database reimport.