This may or may not be useful, but initially reading the term “dev environments” was confusing. It wasn’t immediately obvious to me that a dev environment can be a production environment. Calling it “environments” makes more sense in this case, just as the URI end point refers to it “https://developer.twitter.com/en/account/environments”
Additionally, I prefer in-line documentation/controls for the environment setup page to suggest that I can create more environments by upgrading my account. I wasn’t aware that environments are limited to the account as opposed to one per app with the sandbox subscription. As an example, the subscriptions page (https://developer.twitter.com/en/account/subscriptions) makes it clear there by having upgrade controls.