MacOS – Without a backup, can I downgrade from Sierra to El Capitan

To be frank, I wouldn't worry too much about it. You have plenty of space left. My IconServices folder was about 4 GB large and I had 3 GB of space left on my 160 GB SSD.

I can't tell you why Apple is creating or caching those files on OS X, sadly it seems to be a kind of mystery. I wasn't able to find out anything.

Anyway, after booting my Mac in safe mode (holding down the shift key on startup until the progress bar appears) and then booting up again, I ended up with 13 GB of free space. Try this to free up more space.

If you're looking for a Terminal solution, you can try this:

diskutil eraseVolume free %noformat% disk0s2

or

diskutil eraseDisk free %noformat% GPT disk0

In this case, disk0 should be changed to match whatever your actual device node is. It will depend on whether you're using a USB stick to boot into recovery – in which case it's probably disk1 – or if you're actually booting off of the disk you're trying to erase.

And an important disclaimer: If you blow away your disk, you will not be able to boot into the onboard recovery mode. In fact, you probably aren't going to be able to do this without booting off of USB. I hope you have a recovery stick somewhere, or another computer from which you can make one.

It should go without saying that you will lose data if you run either one of these commands successfully.