Earlier quoted context omitted.
I meant like a filename convention, and tags in the file itself.
Presumably you can look at the file and tell which mode is used, though why would you care to know from the filename?
There are so many reasons why it's almost hard to know where to begin. But it's basically the same reason why it's helpful for some documents to end in .docx and others to end in .xlsx. It tells you what kind of data is inside.
And at least for me, for standard 24-bit RGB images, the distinction between lossy and lossless is much more important than between TIFF and PNG, or between JPG and HEIC. Knowing whether an image is degraded or not is the #1 important fact about an image for me, before anything else. It says so much about what the file is for and not for -- how I should or shouldn't edit it, what kind of format and compression level is suitable for saving after editing, etc.
After that comes whether it's animated or not, which is why .apng is so helpful to distinguish it from .png.
There's a good reason Microsoft Office documents aren't all just something like .msox, with an internal tag indicating whether they're a text document or a spreadsheet or a presentation. File extensions carry semantic meaning around the type of data they contain, and it's good practice to choose extensions that communicate the most important conceptual distinctions.