For reference, a list of applications I have found that don't support files in the WebP image format.
Instagram (2024-12-06): you can't post a WebP image to Instagram.
Gumroad (2025-06-16): you can't use a WebP image as the cover of a product on Gumroad.
Youtube (2025-08-01): you can't use a WebP image as the thumbnail of a video on Youtube.