Webp is an optimized format and widely adopted (https://caniuse.com/webp). I don't know why itch.io doesnt have support for it but it should.

The itch.io editor for game pages should allow webp for adding screenshots, adding thumbnails and images in the "Details" tab.

Webp would especially be useful to use as a replacement for gifs, as gif format is not that well optimized. Also, adding support for webp would also lower bandwidth usage and increase conversions for games too, since users are more likely to bounce if a site takes too long to load.

It would be nice.

I agree, this would be a big improvement. Currently you get:

Your request could not be completed

You selected an image type we don't recognize. It's possible it has the wrong file extension for the format it is saved as. Please use an image editing program to convert it to a PNG, JPEG, or GIF.

WEBP is generally more efficient than all three of the above formats.