If the game is already officially-available online for free (for example, fangames such as Super Smash Bros. Crusade), then linking to an official download is perfectly fine as that is not piracy in any way since the content does not require that you pay for it first. However, if the game is only available commercially (for example, a game such as Super Mario Odyssey), then you may not link to free downloads and may only post officially-supported methods of purchasing said game, such as the official storefront or a listing on Amazon.
TL;DR: Fangames and the such are fine, ROMs and ISOs are not.
Yes, because we don't want this forum to be a piracy forum. Nintendo would nuke it from orbit
There are a ton of other sites so it's not like it's hard to find it.