This is one of the few places where the USA has a sensible solution. In 2015, my org (Themade.org) worked with Archive.org, MIT, the EFF, and Harvard Law to obtain a DMCA 1201 exemption for legal DRM circumvention for the case of single player games where online validation servers have ceased operations.
Thanks to the hard work of these organizations, the US market, at least, allows for those who purchased these games to continue to play them with a third party patch to their client. See below:
https://www.eff.org/deeplinks/2015/11/new-dmca-ss1201-exempt...
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