I agree it is stupid to sign Historic 6 on current 6 (unless for short bits linking sections off US6, such as that brief bit leaving Davenport). But it is signed, so surely we include it?
Not as long as it's entirely redundant, because we don't know where the endpoint should be.
The next waypoint after the signs run out? It might not be massively accurate, but it's better than nothing. We don't want to throw in points for city/county line because one city/county decides to sign and one doesn't - easier just to end it at the next highway. And the end points are arguably state lines, the current route, or a parallel interstate (depending on what state, what route, etc), just with large sections that aren't signed that we're not including.
Or we could go with a logical terminus concept - so, in the absence of info to the contrary, the route stops at the next major junction (similar to what you have asked for in OK extending routes to a more logical terminus like another route) rather than some minor road that happens to be the next point along.
All of those pictures, save for one that's two miles away, are at the same intersection. Historic 6 is not marked with reassurance signs elsewhere in the county. It's a bare-minimum required for the one turn 6 makes there.
True, they merely sign one interchange well, and put one other sign, but - as you say - they clear that bar of 'bear minimum'.
I was replying to "I believe only Cass and Poweshiek counties specifically put up Historic 6 shields where it overlaps the existing route." and said that Pottawattamie County "signs it along vanilla US6 (at least partially)" - that's surely undeniable.
Additionally, I don't see why it has to be signed loads. Especially as you take the opposite view for US66 and asked for it to extend through 4 TOTSOs with US281/US281Spr despite just one sign for all that navigation - a reassurance one just off US281Spr (where El Reno's section begins):
OK US 66 Weatherford: Again, wondering about the abrupt stop for the east end. I would encourage extending this to join El Reno's Hist 66.
But I'd be fine with chopping historic routes at their parent when there is a long signed (at least partially) concurrency with them, despite the existence of signs. Ditto not including sections entirely concurrent with their parent. I'd prefer to include such sections, despite them being silly, but...