July 22, 2011 is the magic date.
command-R will let you boot Macs newer Macs than this date, and so will command-option-shift-R
Prior to this date, a firmware update may be available to enable command-option-shift-R.
Before running recovery, backup your data, make sure it is migrated to your newer machine, and make sure the older machine is deregistered from your AppleID.
Then you can use Disk Utility to either wipe the data partiton, or the entire hard drive, so it is available for the newer user. Inform the new user they may need an ethernet connection to restore the operating system.
When using internet restore on a Sierra or older machine, wipe the hard drive HFS Extended GUID, as APFS formatting is not recognized by the older operating system.
Older Macs you need to find the original installer discs and distribute the computer with them.
Option key boot with the disc, will let you at least run Disk Utility to wipe the hard drive and leave it open for the new user to install the OS of their choice.