Shouldn't it be possible to decompile from wasm to the language that was used to compile to it (or any target language that could've produced said wasm)?
Yeah, but is that actually required? I don't know much about this, but I'd guess 90% of websites will strip this just like debug info in most proprietary software, 9% don't know about this or forget to remove it, if it's included by default and 1% consciously decides to ship it for transparency.
It depends heavily on the specific optimizations and obfuscations used, whether or not debugging information was left in, etc.
There's a reason the GPL has a "preferred form for editing" clause.