If the DRM is enabled, the game does a simple "Is the game available in the user's library?" and steams says yes or no.

If the game didn't have DRM enabled, no check is made. Copy the game folder elsewhere, without steam install and it should launch.

Devs can enable the DRM afterward, but your copy won't be locked.

But even then, if valve goes bad guy, the DRM is simple enough to be broken, and there is no double check or something preventing you from playing (unlike Denuvo which encrypts the game and has multiple separate checks for the DRM).