I haven't read all the responses, but I have a few thoughts:
My mother got rid of a lot of stuff when I wasn't home, and I never noticed (I only realized it when I became a mom that the house had always seemed super-clean when I got back from vacations with my dad!), but I really want my daughter to develop the skill of getting rid of things, so I *try* to do a lot of decluttering with her input, but occasionally I don't. For example, her school is very big on art projects (a lephracaun (sp.?) trap, etc.), and she works so hard on them that I know it would be hard for her to get rid of them, so I take pictures of them, then put them in the outside storage space until I'm pretty confident she has forgotten about them (if she hasn't noticed them gone), then I get rid of them.
Other ways I work on it with her:
One of our babysitters talked with my six-year-old when she was about three or four about how if she got rid of some old toys, there would be room for some new toys, and that really made an impact on her, so that's how I phrase it sometimes, "Are there any toys you don't play with any more that we could get rid of so you'll have more space for your birthday/Christmas presents?"
We know a few younger kids now (younger siblings of her friends), so sometimes I'll say, "It seems like you don't play with this truck anymore. Do you think Delilah would like it?" It seems like it's easier for her to get rid of things when she knows they are going to someone else. We do that with clothes and books too.
I have a lot of friends who "rotate" toys. They just pack up about a third or two thirds of the kids' stuff and put it in storage, then rotate it back out when the kids get tired of what's out.
Sometimes, I take things I know she doesn't play with anymore but that she might have trouble getting rid of - I take that thing and store it in our backyard storage shed for six months or so. If she hasn't noticed it gone and it's something that she truly seems to have grown out of, I get rid of it then.
One big motivator for me was reading the chapter in the book Simplicity Parenting on clutter. The author makes an extremely convincing case that clutter really stresses kids out, whether they realize it or not, so we are doing them a big service when we clear it out for them. Here's the link to the book:
https://www.amazon.com/Simplicity-Pa...city+parenting
Good luck!