I know ROMs are abandonware are generally illegal because the game developers still hold the copyrights. While the law is rarely enforced in this case, most websites do not allow them to be posted.

That having been said, some game companies are known to release their old titles as freeware once they no longer sell. So I'm just curious, have any companies ever released their SNES games as freeware, or even explicitly said that users could download the ROM?

Nope, except for Public Domain games.

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Pretty sure the licensing under Nintendo likely forbid that ever happening.