URLs are case-sensitive for anything after the domain name, so sadly what you’re looking for is not really a Netlify limitation as they’re 2 separate URLs.
What you’re looking for is possible using Edge Handlers. You can check for the URL, convert it to lowercase and redirect accordingly. But, it’s still in invite-only stage.
What you can do is, create a page on your website to redirect to and redirect using JavaScript. So, you can create something like, yourwebsite.com/redirect/?=github (case-insensitive) and using JavaScript, get the query parameter, convert it to lowercase and redirect accordingly.
If you don’t want client-side redirect, you’ll have to add all combinations of URLs to your _redirects.