boundary=historic didn't affect the Nominatim addresses, and your change-set was labelled "Remove place=county from historic counties in Northern Ireland, improving Nominatim results".  I just question your logic/reasoning of removing tags from boundaries, without asking or discussing it first.  I wouldn't think about going into an area and mess with the tags, to "fix" addresses on Nominatim, because I don't know the area.  There are plenty of active mappers you could have messaged and asked.  There are plenty of businesses with address listed on websites to get an idea of what happens on the ground.  