As a followup to my note in August on this topic, here’s what Tom Greenwood had to say about WebP’s value to sustainability in WordPress:
I think as most browsers support WebP now, there is a real benefit to serving this format as it saves a lot on data transfer and improves performance. The downside perhaps is that most images are uploaded as JPG and pretty much all implementations that I have seen will then generate the WebP files as duplicates, so there is unfortunately an increase in data storage. I think the trade-off is worth it though.
As for SVG, these can be way more efficient in terms of both data storage and transfer compared to JPG or WebP, but of course they should be well optimised, and I’m not sure of an automated way to do that, so there is more hands on work there. The downside here is that complex SVG files could cause more CPU energy to be used on the end user’s device, so the simpler the design and more optimised the files the better.
