Collared cats face the same risk as "lanyarded" off trail hikers ... becoming hung by the neck.
You can buy breakaway collars like these, the one on the left came from PetCo ($8 and weighs 8 grams) and the one on the right from Target (approx $4 and 9 grams)
cut down they weigh in at 3 and 4 grams respectively:
Attached to a strap made from 1.1ozDWR nylon from a Thru-Hiker project it makes a lanyard weiging in at 6 grams
A key ring adds 4 grams
Here it is broken away
and as a bonus, I'm left with these SUL bear bells (LOL)