Letting kitten out in the garden

[NotABeliever]

Hi everyone
I'm after some advice as a very inexperienced new cat owner! We got an adorable Russian Blue kitten a few weeks ago and been advised by the breeder that they are indoor cats and should not be let out un the garden. She's now almost 6 months old and she's never been out. We're terrified at her escaping and we pay a lot of attention at keeping doors and windows closed when she's in the room which is hard in this warm weather.
Is this necessary? My brother has a cat and he seems to think it's nonsense. He thinks she should be allowed to go as she pleases but we're terrified she'd get lost of someone may steal her.
This is she having a little nap 😊
Any advice greatly appreciated

There are some sick people out there.

Agreed. The things I have seen I wish I could wash my brain of.

Keep your cats inside or supervised in your yard or on a leash for walks.

My cat is an indoor and we let him in the garden on a harness and lead. He does jump and tries to get into next door so we have to watch him.

I have an 8 month old blue burmese- she looks a bit similar! (not relevant to going out). We kept her in until we had her spayed last month (a bit late due to lockdown). Now she's recovered, we have the doors and windows open, but she really doesn't like it outside! She'll go in the garden if a human is out there, but otherwise she's extremely cautious and barely leaves the back step. I feel better that she has the option though.

@BeefCheeks I can only speak from my own experience, which is why I specified what cat I have and her personality type.

OP needs to assess her own situation and determine what is right for them. If Blues have a tendency to get lost then that is a big factor to consider.

Thanks for all the advice!
Just to clarify, the contract with the breeder doesn't say anywhere that we need to keep her in. It was just the advice she gave us and she keeps all her Russian Blues as indoor cats.
I hadn't realised that this breed is particularly bad with sense of space a direction. It's very useful to know. Thank you all.