What to keep, how to keep it, what to release.
Keeping everything and keeping nothing send the same message. There is a middle.
Volume is the practical problem, and it arrives faster than anyone expects. A child who works often produces more paper in a month than any house can absorb, and the two obvious responses both misfire. Keeping every single sheet in a mountain on top of a wardrobe means nothing is ever looked at again, so the work is effectively discarded, just slowly and with more storage involved. Quietly binning everything, particularly where it can be found, tells a child their work is rubbish, and children do find it. The workable position between the two is deliberate selection, done openly rather than in secret, with the child involved often enough that they understand the system and do not feel robbed by it.
A very simple method holds up over years. Have one flat box or folder per year and nothing else. Photograph far more than you keep, on the day it was made, laid flat and in decent light, so that the work you do not physically keep still exists somewhere and can be looked at later. Keep a small number of actual sheets from each stretch of time, chosen for range rather than for quality: one from a day of pure scribbling, one from the week a new symbol first appeared, one enormous one folded down to fit. Write the date lightly on the back and nothing else at all. Do not annotate it with what you think it shows or what the child said it was.
Then use what you keep, because storage that is never opened is just a slower bin with better manners. Put a few up where they are actually seen and rotate them regularly rather than building a permanent gallery, which turns display into a prize and makes the absence of a sheet a verdict. Look through the box together occasionally, without any commentary about progress or improvement, and let the child notice their own changes in their own time. And then let the rest go without ceremony and without apology. The sheets were only ever the residue of the activity, the activity itself already happened, and the child took everything that was in it at the time.