If it was a violation then people with the minor magical talent of psycometry would be in trouble a lot.
The Gatekeeper possibly sends messages to himself from the future and nobody asks him if he's breaking any laws. Doesn't even ruffle feathers as long as he keeps his use of that knowledge subtle.
Wizards develop visions of the future if they live long enough which is arguably information traveling backwards. They can't even help it, as it's just a natural development rather than an end sought after by spell work.
And as has been pointed out, the knowledge is out there already, either in books, the Akashic record, witnesses in the form of long lived entities, etc.
So yeah, scrying into the past is pretty unlikely to break any laws.
If Cold Days is right then it's damn near impossible to break that law anyways, probably harder the further back you go.