I don’t intentionally use it to keep track of the games I’ve played but I’ve been using GameFAQs to keep track of older console games I have not yet played. It’s not perfect for this as I need to use documents to keep track of some additional information but I have yet to find a better alternative.

For PC games, some of the platforms I use, like GOG and Itch, actually have built in features to help me to keep track of what games I haven’t played yet. For the others (and mobile) I still have to use documents to keep track of this as well.