I think this change was misguided and should be reverted.
Counties Down, Antrim, Armagh, Fermanagh, Derrylondon & Tyrone are most definitely counties, and should remain tagged as such, 'place=county'.  
It is true that (since 1972) they are little used for administrative purposes (not for voting or collecting bins or organising fire brigades or in official Royal Mail postal addresses** or etc etc*) - hence the 'historic' tag. But they are very much still known and used by the people who live in them, in what I guess we might call a 'geographical' sense, simply as places. 
I don't really know much about 'Nominatum', but if it is outputting things like 'Enniskillen is in County Fermanagh' or 'Milisle is in County Down' these are perfectly sensible, useful and correct things to say.

*one thing they are still used for is 'Lord Lieutenancies' for the UK monarchy. https://en.wikipedia.org/wiki/Lieutenancy_area

**adding a NI county to a postal address is not Royal Mail's official way to do things, but is not uncommon, and is never going to result in a letter or parcel going to the wrong place. Putting the current NI district council names in postal addresses is pretty NEVER done by anyone and Nominatum should certainly not be doing it! 
Another geocoding faux-pas is to add to mistakenly add the county of an addresses *Post Town* to an address - for example, many County Down addresses outside Belfast have 'BELFAST' for their post town - and it is very incorrect to instead put their address as 'BELFAST, County Antrim'. 