Question

Restaurant was very crowded, we decided to stay.
Is this a run on sentences

Answer 1

yeah, i would think so, bc they just put a comma and kept on goin with sentence instead of dividing it into two seperate sentences like it should be

Answer 2

I don’t think that this is a run on sentence.
An example of a run on sentence is

Run-on sentences, also known as fused sentences, occur when two complete sentences are squashed together without using a coordinating conjunction or proper punctuation, such as a period or a semicolon. Run-on sentences can be short or long.

According to https://www.grammarly.com/blog/run-on-sentence-basics/

In the sentence
Restaurant was very crowded, we decided to stay. It has a comma

Hope this helps :D