This has nothing to do with Steam (as far as I know it was never on Steam, nor planned to be). The project's git repo [0] is down, and he implies he will not be working on it anymore. He has also said [1] that he will be taking down the ROM download (patcher) [2] soon, as well.
[0] https://github.com/lambertjamesd/portal64
[1] https://twitter.com/JoeyCheerio/status/1745150271230038228
[2] https://lambertjamesd.github.io/RomPatcher.js/index.html