My experience with slots - I play mostly those that are marked as popular. I have never had a favorite slot that I have market as favorite and played only it on purpose. However, I dont know what really is old slots. Slots seems to be identical, and there are so many different slots by design, that all of them look «new» to me. To be honest, even old looking slots or classics lemon/777/watermelon/plum/cherry/peach are still popular and are active. I think that means there are almost no old slots.
In my opinion we just need to separate the age of the slot itself from its design. The classic fruit-machine style doesn’t really tell you how old a slot is. Providers simply combine the old-school style with modern graphics, bonus rounds, and new features.
But at the same time, there are actually quite a lot of genuinely old slots, and some of them are still popular today.