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Yes. Although such situations are only really likely to arise where there is no pavement and walking wouldn’t be any safer than riding. It’s a little unclear if you can ride an illegal road if it’s no longer of the classification that makes it illegal. I stumbled across an incomplete motorway which I was told by a worker I could cycle on. Best course is take your best judged course of action and document it.
GoatandTricycle-
To stick within the rules you can't go on what locals tell you - even the police. In 2016 @Swissy was penalised for using a motorway in Macedonia that the police told him he could use. IMHO he didn't get a big enough penalty as he missed the slow, gravel road and saved a night, so went from being level with me to finishing about 12 hours earlier! ;)
I guess in that situation, if the road has been officially downgraded and the motorway signs have gone, but the maps haven't yet been updated, it would be ok.
frank9755
Surely for such short stretches (maybe not in a tunnel) one could quite legitimately just get off and walk, anyway?