For a long time, this was the clarification people were going by:
Clarification on trail markers
NianticGiffard: Please check our stance on the below scenarios:a) A marker with the trail name on the trail ← Excellent
b) A marker with the trail name on a street ← Good
c) A marker with no trail name on the trail ← Good
d) A marker with no trail name on an open green space area ← Good enough
e) A marker with no trail name on the street ← Not Good
Niantic has never formally retracted that statement to my knowledge, but it was made on the old forum and was not carried over to the new. But trail markers are still encouraged in the current clarification:
IMO these signs were submitted in good faith, follow the posted guidelines, and meet the criteria of encouraging exercise and exploration. I don’t see any abuse in submitting them.