Thank you for reaching out and I am happy to help!
These images are not residue after the W3 Total Cache removal. W3 Total Cache image service does not replace the original files with webp images, it uses the rewrite capabilities of the server and adds serves the images as webp.
This being said, there are no webp formats in the library after the W3TC converts the images.
This means that either some other plugin was used to convert the images to webp in yoru case.
As you can see in the screenshot below, the image retained the original format, it was just converted on the W3TC server and served as webp:
Ok thanks, then probably imagify or another plugin I used before.