SIMD.js has been taken out of active development in TC39 and removed from Stage 3, and is not being pursued by web browsers for implementation. SIMD operations exposed to the web are under active development within WebAssembly, with operations based on the SIMD.js operations. With WebAssembly in advanced development or shipping in multiple browsers, it seems like an adequate vehicle to subsume asm.js use cases, which are judged to be the broader cases. Although some developers have expressed interest in using SIMD.js outside of asm.js, implementers have found that implementing and optimizing for this case reliably creates a lot of complexity, and have made the decision to focus instead on delivering WebAssembly and SIMD instructions in WASM.
See https://github.com/WebAssembly/simd for current development.
This repository retains a historical snapshot of the SIMD.js specification work: