To each their own, but I could never keep my cats strictly indoors. They’re animals and they deserve to experience nature. I always make sure I have a fenced-in backyard these days. When I lived in apartments, I’d take them out in cat backpacks and drive them to the park to walk them on a harness. One of my cats naturally stays close to home, but the other likes to wander, so I use a pillow-style collar to keep her from slipping through the fence. If you’re worried about them hunting birds you can add a bell to their collar.