Earlier quoted context omitted.
I've submitted a correction for an illegal turn instruction twice on Google maps. No reply. But my other corrections have been accepted.
The way to get that corrected is to ask the local police to ticket illegal turns at that intersection for a day, indicating that Google refuses to correct the issue and is directing hundreds of violations a day at the intersection. Google ignores unenforced road signs that are frequently ignored by their data providers (drivers), so enforcement complaints from drivers are necessary to force the correction.
Their other data providers are me, as a user submitting a correction, and Streetview, which they could use to confirm the turn restriction.