When, exactly, did US 21 disappear from West Virginia?

[Mapmikey]

1972 WVDOH traffic flow map still shows US 19 on/multiplexed with WV 41 from Summersville to Beckley, and US 21 following WV 16 from Beckley to Gauley Bridge and then US 60 into Charleston.

This showed traffic data from 1971, so it would not have shown US 21 on a new Corr L that had not been finished.

The AASHO application to move US 19 to Corridor L was in 1970 and did not mention any interim routings prior to completion of the entire Corridor, nor mention US 21 at all.

[Dirt Roads]

1972 WVDOH traffic flow map still shows US 19 on/multiplexed with WV 41 from Summersville to Beckley, and US 21 following WV 16 from Beckley to Gauley Bridge and then US 60 into Charleston.

This showed traffic data from 1971, so it would not have shown US 21 on a new Corr L that had not been finished.

The AASHO application to move US 19 to Corridor L was in 1970 and did not mention any interim routings prior to completion of the entire Corridor, nor mention US 21 at all.

Actually, H.B. and others was/were correct.  US-21 had been routed over the Oak Hill Expressway since 1961 (which was famously and incorrectly referred to as the first freeway in the West Virginia History textbooks).  So when Corridor L was first shown on the West Virginia official maps, US-21 was indeed shown on the only completed portion of Corridor L (nowhere near the routing of US-19 in those days). 

I can't think of any other instance in West Virginia where one U.S. route gets supplanted by another.

All of which leads us to a corollary of the O.P.  Does anyone know if US-21 was still posted on the Oak Hill Expressway when US-19 was rerouted onto Corridor L after the New River Gorge Bridge was completed on what is now known as "Bridge Day" in 1977?  I travelled on the Oak Hill Expressway numerous times prior to construction of the bridge, but then always went to see the bridge via US-60 while it was under construction.

[Mapmikey]

Actually, H.B. and others was/were correct.  US-21 had been routed over the Oak Hill Expressway since 1961 (which was famously and incorrectly referred to as the first freeway in the West Virginia History textbooks).  So when Corridor L was first shown on the West Virginia official maps, US-21 was indeed shown on the only completed portion of Corridor L (nowhere near the routing of US-19 in those days). 

He was discussing the part of Corridor L north of the original Oak Hill Expressway but south of Fayetteville.  There is a topo map that labels this as WV 16 Byp and US 21 is nowhere to be found at all.

[Dirt Roads]

Actually, H.B. and others was/were correct.  US-21 had been routed over the Oak Hill Expressway since 1961 (which was famously and incorrectly referred to as the first freeway in the West Virginia History textbooks).  So when Corridor L was first shown on the West Virginia official maps, US-21 was indeed shown on the only completed portion of Corridor L (nowhere near the routing of US-19 in those days).

He was discussing the part of Corridor L north of the original Oak Hill Expressway but south of Fayetteville.  There is a topo map that labels this as WV 16 Byp and US 21 is nowhere to be found at all.

Yep, I believe that this was also mentioned in the <Without the Interstate System> thread.

I've got a 1973 DOH Functional Highway Planning Map that essentially takes the "official" state map and overlays highlighter colors for each major category.  Beneath the dark green highlighter you can still see the segment of Corridor L between Oak Hill and Fayetteville under construction.  But like you example, all of the US-21 labels between Beckley and Charleston have been removed.  I'm guessing that the DOH was trying to discourage folks from using that route; not so much to force traffic onto the Turnpike but more to keep from loading extra traffic onto US-60.

This 1973 map is interesting because it shows planned "Expressway" (dark green) across the yet-to-be-complete New River Gorge Bridge, but zig-zagging back over to the current-day WV-41 corridor.

[Dirt Roads]

^^^
I also have a 1975 Highway Base Map from the DOH that shows US-21 on the old alignment north of Oak Hill.  I don't ever recall US-19 multiplexed with US-60 from Clifftop -to- Hico, but both this and the 1974 official maps show US-19 rerouted onto the new Corridor L alignment north of Hico.  (Given the history of the DOH, US-19 may have been posted on both the WV-41 multiplex and Corridor L north of US-60 until the New River Gorge was completed.

Edit:  The 1975 Highway Base Map shows Corridor D, Corridor E, Corridor G, Corridor H, and Corridor Q in a sideways oval on the route.  In some cases, the affected U.S. routes had not been rerouted by then.  Corridor G is shown "multiplexed" with WV-65 [west] of Logan. and without a route number between Chapmanville -and- Madison.

[Bitmapped]

All of which leads us to a corollary of the O.P.  Does anyone know if US-21 was still posted on the Oak Hill Expressway when US-19 was rerouted onto Corridor L after the New River Gorge Bridge was completed on what is now known as "Bridge Day" in 1977?  I travelled on the Oak Hill Expressway numerous times prior to construction of the bridge, but then always went to see the bridge via US-60 while it was under construction.

The 1976 WVDOH General Highway Map (regular state map) does not show US 21 at all. US 19 is indicated on its present alignment north of US 60, with the route through Nallen shown as WV 41. Shields for US 19 and WV 41 are both present between US 60 and Beckley.

There are 1969 and 1976 versions of the Oak Hill and Fayetteville topo quads. Interestingly, none of them show US 21. The Oak Hill Bypass is marked as Bypass WV 16 and the other completed part of Corridor L south of Greenstown, where WV 16/WV 61 split off onto the old alignment today, doesn't have a shield. The quads show WV 16 and WV 61 basically on their present-day/historic alignments.

Things get interesting on the WVDOH maps. There aren't specific shields shown on Corridor L, but on the 1976 map, it looks like it's the only state route through the area. It doesn't show the parallel WV 16/WV 61 alignment from Glen Jean to Greenstown/Oak Hill. It makes it look like those routes are on Corridor L.

The 1980 WVDOH map clearly shows WV 16/WV 61 on their current alignment south of Greenstown, although it looks like they follow Corridor L up to the full diamond interchange at Main Street versus leaving at the half-diamond at Pea Ridge Road. WV 16 is back on its own alignment between Oak Hill and Fayetteville. This map makes it look like WV 16 doesn't briefly rejoin US 19 north of Oak Hill, even though it definitely did because part of Corridor L was built on top of the existing WV 16 alignment.