Whoa.
I thought of something.
Let's say a stock spring is 12" tall. Add a 2" spacer. OK.
Going extreme, say you use a 7" lift spring for 19".
Now, knowing how coils work. When compressed they want to spit out sideways.
Squeeze a pen spring.
Add a spacer, just adds another seam to fail. Something else to fall out of place. Adds looseness.
Works on a small stock spring, may work on lift springs, but not advised as it could possibly attribute to failure. Not that it will or will not.
Not RECOMMENDED.
Thinking of the springs bolted on the bottom and loose on the top, if your suspension flex enough to allow the spacer to fall out.... ooh. Trouble.
Sure, possible to flex stock. But unlikely. Lift suspensions usually flex way more than stock.
Either way, I get it now.