Any non-real money online casino game that you play with the intent of simply practicing is a demo game. Players routinely play demo games to fine-tune their gaming tactics, techniques, and strategies.
A major factor in the decline of the demo is the advent of early access for titles on PC. With so many games receiving some degree of crowdfunding in the modern era and Steam's policy of allowing developers to put games on early access, games are in a perpetual demo state.