The 20-30% rule was originally for hard disks. Once you reached that number the number of seeks to find open blocks on a highly fragmented drive would start to slow the system and could cause a system crash if the disk started thrashing.
With SSD's it is more complicated with effectively no seek time time and over provisioning to keep cells from wearing out. The greater the over provisioning and free space the better the performance and lifespan of the SSD, within limits. Not totally sure how to balance the provisioning and free space numbers on an SSD so I just fall back on the old rule just to be safe. Assume you could reduce the free space requirement if you have a high provisioning number.
As the cart below shows you can get 60% better performance with 20% vs 0% provisioning. The colored lines are for different SSDs.
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