Is a speed limit sign a warning sign?

The correct answer is NO. A stop sign is considered a Regulatory Sign.

Regulatory signs are typically the colors black, white, and/or red. Regulatory signs regulate traffic and tell a driver about a law. Examples include speed limits, yield signs, one way streets, do not enter signs, no turning, no parking, and (yes) stop signs.

A warning sign, on the other hand, is typically diamond-shaped and the color yellow, orange or pink. They warn you of possible changes in the use or condition of the roadway ahead.

A stop sign is not warning you to stop. It is TELLING you to stop because that is the law.