save-game isolation is important.
- Some simple engines don't allow the author to change the save "id" and those saves clash between different games.
- localStorage is limited per domain (5 or 10 MB depending on the browser) so one game can reach the limit and cause issues for all other games
- if one clears the localStorage to solve an issue with a game, it clears it for all games
Vanity URL are indeed not needed but somewhat unique URLs are.