Is it permissible to play a copy of a game not protected by copyright, knowing that its modern versions are protected?

It is not permissible to resort to stratagems to download or play games that their producers prohibit doing so except by purchasing them, because that constitutes an infringement on their rights, which are legally protected. However, if it is established that the older part of a game is no longer copyrighted and has become public domain, then there is no harm in playing non-original versions of it.