No bots. Just humans.
Reviewer's decisions are final. There is no appeals process, sorry.
With over 300 million assets in inventory, Stock is more selective about what they accept now. And the quality bar has risen.
Compare your work with current Stock inventory to determine if Stock needs more representation of that subject or not. As an example, Stock currently contains over 29 million Flowers-- far more than demand requires. The same is true for Abstracts which currently exceed 52 million.
The current challenge for contributors is finding new and unique subjects and closely examining them beforehand to ensure they meet the highest visual & quality standards. It's not as easy today as it was previously.
I hope that helps.