Similarity search has been added to Flickr:

1. Make an initial keyword search inside Flickr.

2. Select a photo thumbnail, hover your mouse over it.

3. A three-dot bar will fade up in the top-right corner.

4. Click once on the three-dots and select Search for similar photos.

My test with a ginger kitten on a green background suggests that only basic parameters are being searched for. In this case “cat face with ginger fur”, rather than “cat face with ginger fur” + a green background. Nor is the search able to distinguish between grown cats and kittens, which suggests no keywords are being added to the mix.