Not only.People also abandon cats when they move to a place that doesn't allow pets, or feel they can no longer afford to feed the cat, or they get a new baby, or they claim a new allergy, and other reasons. A domesticated cat, dumped in a feral cat colony can be a bad thing for the domestic cat.
Most shelters are full this time of the year b/c it's kitten season. They should free up more soon.
Also because of the CXXXXX event, a lot of people took pets in and than just change mind or life change. All shelters around me full to capacity and pretty much begging for adoption.
Back-to topic: Buy the special friend food or treats (separate from your cats). Have a blast with somebody that listens to you. Still guard your indoors.
And yes: You CANNOT make an outside cat into a completely indoor cat. Can't compare the hunt+all the smells and stimulation from being outside.
Just watch them after the rain....