Definitely depends on the cat. We have two rescues, wanted indoors as we live on a busy main road. Got the first one and she’s very content indoors. She was 18 months when we got her and had had a very rough life. She likes her home comforts.
Got the second a year later, who was 3 months old. Was reassured by rescue he was suitable for indoors, (looking back no way they could have known that!). Tbh they were struggling to home him I think so were glad we were looking again (same rescue).
Well the cat is part Maine coon (unbeknown to anyone when we got him!) and DETESTS being kept indoors. We started letting him out after 6 months as he really seemed unhappy.
He’s been going out for 3 years now and is such a happy cat. He’s always out and only comes home for food and a fuss (only on his terms though!) or to present to us whatever he’s caught 😳😳
Our first little princess has the choice to go out also, and never bothers.
So I don’t think it’s cruel to keep a cat in if they’re happy. But it would have been cruel for us to keep ours in, because he clearly was desperate to be outdoors.
(Thankfully we have woodland behind the house and a huge garden, he doesn’t seem to venture round the front of the house thankfully and is content is terrorising the woods!)