I recently sold my M3 Max MBP 14 to a buyer and learned from Apple Support that I can't transfer my active AppleCare+ plan to the new owner.
Apparently, if I had purchased the 3-year plan upfront then I can transfer to another person (which I've done before with another MacBook). But this time around I had elected for the yearly AppleCare+ plan and that's apparently different and locked to my AppleID for billing purposes, and cannot be transferred to someone else. The laptop is still covered for the new owner but they won't be able to renew the plan themselves, unless they pay me to renew it on their behalf each year or just let the plan lapse.