UPDATE: This from Mike...
That's just pathetic, especially since there's an official sign!
This may interest you, a website that shows all the different rail signs, including the 5-10 different HST Stopping markers.
UPDATE: This from a Mr Chris Milner...
Actually, the sign does have a point, as the yellow NMT visits these platforms at Euston, and is the only HST regularly active on the WCML.
UPDATE: This from @Gav678, via Twitter...
Your pointless sign at Euston is most certainly not useless for the NMT.