Under about 6, just get rid of stuff and if they ask about it be like "Oh I'm not sure, I haven't seen that for ages either" they will forget about it in 5 minutes anyway. If they keep asking about something persistently, you can always get another one or a new version.
You can also instigate a quarantine area, where you tidy something away into a box or bag and hide it somewhere for 6-12 months, if they don't ask for it in that time let it go. If they do ask for it in that time, you can get it back out again.
When you are thinking about what stuff to get rid of, I would definitely base it on observation of their play, rather than just what you think they should like. Notice the stuff they pull out and cast aside to get to other things. Also consider how your toys are stored. I find that perodically getting everything all together and then separating it into categories by type of play is the most effective. So for example, we had a load of animal/dino figurines and they were just shuffled in with everything else, so they became "stuff to dig through" when the kids wanted a car or a truck or something. I took everything out and put all the animals in a tub with some fences and suddenly they are making play farms (with dinosaurs and kangaroos in 😆) I also got rid of the random 1-2 pieces of fence which didn't attach to anything else, as they never played with those.
If you have toys that only work when all the pieces are present, like a shape sorter, or jigsaw, things like that, again, the pieces on their own will just become "stuff to dig through" and contribute to the mess. So go through and locate all those pieces and then put them inside the main toy, store that somewhere that they have to
For older children, start involving them in decisions but don't frame it as "Which toys shall we get rid of?" because that invokes a feeling of defensiveness instantly and they will come up with a million reasons why everything is their favourite. Instead, frame it as "Shall we make some space so you can get to your toys more effectively?" and then "Which are the best ones to fit in this space?" from there you'll be able to see what they actually want to keep and they can see what actually fits, and it's then easier to say "Shall we give these ones to some other children who don't have any toys of their own?" or even "Shall we sell these for some money for your piggy bank?"