Nothing screams "future proof" like WASM.
Is there any reason to believe that a major new browser tech, WASM, will ever have support dropped for its early versions?
Even if the old versions are not as loved (i.e.: engine optimized for it and immediately ready to be executed) as the old versions, emulation methods work wonders and could easily be downloaded on-demand by browsers needing to run "old WASM".
I'm quite optimistic for the forwards-compatibility proposed here.
However I don't think it matters too much. Here WASM is not targeting the browser, and there are many more runtimes for WASM, in many diverse languages, and they outnumber browsers significantly. It won't die easily.
<blink> is gone. In C, gets is gone. It may take only one similar minor change to make huge amounts of data unreadable by newer WASM versions.
Depends how obscure. You can play Z-machine games on pretty much anything now.