I recently found some packs of gunge/slime powder that I had forgotten about, they must be about 10 years old and have never been opened. What's inside doesn't appear to have gone hard or lumpy and is still mostly powdery from what I can tell, but I'm just wondering whether it will still be any good and, well, do what it's supposed to do?

I don't want to get my hopes up only to find that it doesn't work as intended and find that I then have nothing else to enjoy ourselves with, but on the other hand I don't want to buy "additional supplies" only to find that they disappear into a cupboard and get forgotten about too.

Generally if its been stored in a cool dry place there should not be any issue and it would mix fine as its the suspension in liquid which 'activates' it

Mix a small quantity from one of the packs and see.

Dry powders usually store very well if they're kept in a sealed container in a dry location.