Comment by jonsneyers

PNG can and is often used in a lossy way. Reducing the number of colors so PNG8 can be used instead of PNG24/PNG32 is the most common way to do that. Tools like pngquant exist, and for example Photoshop when exporting to PNG also has an option to reduce the colors, to flatten the image (remove alpha), or to change the colorspace.

16-bit PNG files can easily accidentally be reduced to 8-bit, which is of course a lossy operation. Animated PNG files can easily get converted into a still image (keeping only the first frame). CMYK images will have to be converted to RGB when saving them as PNG, which is also a lossy operation. It can happen that an image gets created as or converted to JPEG and then gets saved as PNG - which of course is a bad and lossy workflow, but it does happen.

So I don't agree that with PNG there is no way to accidentally make it lossy.

In any case: lossless or lossy is not a property of a format, but of a workflow. For keeping track of provenance information and workflow history, I would recommend looking into JPEG Trust / C2PA, which is a way to embed as metadata what happened to an image since it was captured/generated. Relying on the choice of image format for this is fragile and doesn't allow expressing the nuances, since reality is more complicated than just a binary "lossless or lossy".