Doppler (https://doppler.com) is my preferred tool for storing API keys. It centralizes where you manage all of your environmental variables and makes it so you never risk exposing your API keys in a code repo. There's a CLI tool that makes it easy to use all of your environment variables while you're developing and a ton of integrations for wherever you prefer to deploy your code. Feel free to ask me anything about Doppler or secrets management - I'm a developer advocate at Doppler.

Check out Infisical: https://infisical.com

We help developers manage API keys, DB access tokens, certificates, and other types of secrets across all of their infrastructure – providing smooth native integrations with k8s, terraform, github actions, any local development setup, and much more.

Disclaimer: I'm one of the founders.

i have a private repo with a script that backs all my .env* files up and then encrypts them and pushes them to github.

hashicorp Vault