> This was a value not in use

This is untrue - as you would have seen had you looked at https://taginfo.openstreetmap.org/tags/archaeological_site=castle#projects !  

> by the very mapper who set out to restructure the historic=archaeological_site family half a year ago ... Since nothing is left of this castle to be seen (aerial imagery shows a plain meadow), mapping may be questionable in the first place,

Did you ask her about it?  I bet she'd have been able to help :)

The tiniest bit of investigation by me found https://en.wikipedia.org/wiki/en:Threecastles%20Castle , which is linked from https://www.openstreetmap.org/way/553974997 , which mentions old OS maps, which are available in iD, and this is clearly the "other" castle mentioned in the wiki page and clearly visible on the old OS map.

> There isn’t yet a clearly established tagging scheme for former,

Sure there is: https://taginfo.openstreetmap.org/keys/fortification_type#values

> So I specified this, not removing information, but adding information by being more precise.

That is simply untrue.  It was clearly mapped before as "something that used to be a castle".  Now it is mapped as "something that is some sort of fortification, but with no information about the period or structure".  See https://map.atownsend.org.uk/maps/map/map.html#18/53.18182/-6.48529 for the difference that it causes - now the eastern one is just "some sort of fortification" and the western one "some sort of castle". 