Under the expansively vague copyright provisions added thanks to Hollywood lobbyist pressure in the 80s & 90s to deter physical media piracy (Sony Betamax / DeCSS DVD), basically nothing except limited fair use is clearly "permitted."

However, in practice, a lot of historical retro preservation and emulation for personal use is 'tolerated', as long as it's titles long out-of-print, strictly non-commercial (no one makes money) and kept relatively low-key. Well-known projects like MAME and meta front-end RetroArch have existed for decades and done much truly wonderful work.

While manual decomp / recomps existed in the scene, the 2020 release of the amazing Super Mario 64 decomp SM64 showed that hundreds of hours of painstaking work could unlock ports, deep mods, community translations/expansions, and academic study of classic titles. In the last couple years decomp/recomp has been turbo-charged with LLM agent assistance. Of course, there's now the usual spectrum between carefully guided, curated efforts being accelerated and generally unhelpful slop.

As someone into retro preservation for 20 years, the explosion of interest, new projects and contributors has been great but there's also concern that too much attention, too quickly could attract a wave a profiteers who 'cross the line' the community has been careful to avoid, triggering a legal crackdown. Even before decomps, in recent years Nintendo has been growing more aggressive against some retro community activities which were previously tolerated. So, welcome to the community but please try not to screw up a good thing.