I've very rarely seen, say, image viewers which can't handle a JPEG with a .png extension or vice versa, but they're badly designed. Most handle this from the file magic automatically. Keying on extension is definitely an antipattern IMO.

I don't see people naming C++ files .c++03, .c++11, .c++17. I don't see people naming Perl files .pl-strict-mode.

Yes, it's normal for people to use file extensions, but having programs impose forced usage and semantics to them is a crutch and a sign of bad design.

How am I supposed to put a script written using this tool in $PATH?

This bad design seems to be a thing in the TS/JS ecosystem now. At least some language ecosystems (Deno? Not sure.) seem to be encouraging people to do 'import ".../Foo.ts"'. Which is abysmal: Firstly these are URLs, yet file extensions are against web principles and good URL design. Secondly they couple a consumer of a module to what language the module was written in and expose this as part of an API, which is terrible design.