Can I delete /System folders in Time Machine backups to save space?
You can certainly do this. I would first set Time Machine to exclude the system folder and then get a back up or two intervals.

Then you can browse the current backup and step back in time. Once you see a folder show up - control click (right click) and delete all versions of the affected folders.

In practice, system changes are very low and small - so you might only save 20 GB or so. Applications might save a little more space, but you can do what you want.

I generally just set my drive on the shelf and get another $100 drive and start over. Much easier than squeezing the last few GB from a full drive - but if you back up to a network volume - cloning that to an external drive takes quite some time in practice. That's also a nice option if you can spend the time and then start over. Be sure to test restoring a file before you delete anything you might care to not lose.