I’ve checked this for my own sites as well, and get the same result. When making requests, Twitter indeed responds with a 400, with the body including “<h1>This browser is no longer supported.</h1>”.
I am not sure what can be done to make that work — in my opinion, Twitter should not be blocking unsupported browsers and responding with a 400. (At the risk of sounding pretentious: it’s against the spirit of the web.)
As for running the external_links (a.k.a. elinks) check: I recommend not having it as a deploy-time check, as it’s generally flaky (e.g. it will cause your deploy to fail if some other site is down, misbehaving, or responding slowly). The internal_links (a.k.a. ilinks) check is fine and recommended to run at deploy time.
The colon at the end of the URL is not an issue: it is likely VS Code misinterpreting where the URL ends when making it clickable.
[dəˈni] • pronouns: he/him