No, ANC-IAH and IAH-BOS are both in the timetable for December. Both have stops (PHX and PHL respectively) but carry the same flight number on both sectors. They're perfectly legit for use with RTW tickets. You get the city pair miles, not the point-to-point.

These come and go, for example RNO-JFK or SMF-JFK both used to turn up pretty often. It seems like US was/is more inclined to do this; I seem to recall I was on a SEA-PHL US flight (don't remember when, I do it pretty often) where the FAs announced that the flight was continuing on to Europe someplace, obviously with an equipment change in PHL.

They're almost never good in the reverse direction, e.g. IAH-ANC.