Jpeg XL is a new-ish modern image format (ISO standard fully finalized in 2022), with compression rivaling or surpassing current formats. Reference encoder/decoder is libjxl in C++, and decoder jxl-rs in Rust.
It is royalty-free, a free and open standard.

Awareness and usage of it is about to increase in a big way, as support will be enabled by default in Firefox and Chromium within a month. And it is already supported by a wide variety of image-related programs such as Photoshop and Krita etc, and has been announced to be included in the PDF spec in the future.

With support for it I could, losslessly, cut the total filesize of my slideshow wallpaper library that's currently .pngs in half.