.env files are a thing in tons of codebases

but thats at runtime, secrets are going to be deployed in a secure manner after the code is released

.env files are used to develop as well, for some things like PayPal u dont have to change the credentials, you just enable sandbox mode. If I had some LLM attached to my codebase, it would be able to read those credentials from the .env file.

This has nothing to do with deployment. I never talked about deployment.

If you have your PayPal creds in your repository, you are doing it wrong.

.gitignore is a thing

Which every AI tool I’m aware of respects and ignores by default.

Why is it that they can add new env variables then?

It is trivial to append to files without reading them. Also, no AI provider even wants your secrets, they are a liability. Do whatever you want though, I'm not here to convince you of anything.

If your secrets are in your repo, you've probably already leaked them.