in the RequestInitializerDict struct (here), the WontImplement field is set. This causes a hard-crash where libraries would like to support browsers, which require this API, and workers, which makes it illegal to specify at all. It's also very difficult to work around this, as it's specified in multiple places that this should immediately cause an unrecoverable crash.
Related:
developit/redaxios#64
cloudflare/workers-sdk#2514
ethers-io/ethers.js#1886
sindresorhus/ky#366