Since recent versions of the Atlassian application links library, the two authentication types "basic auth" and "trusted applications" are not available anymore. Instead, only OAuth and "OAuth impersonation" are supported.
Is there still a way to use one of the two mentioned removed authentication schemes with application links? E.g., what do you do when you want to link two Atlassian applications, but some users only have an account in one of them but you still want to allow them to access information of the other app where they don't have an account?
The basic auth and trust application link types were removed in version 5.2 of the application link plugin. OAuth is the only authentication type that can be configured after this version. The recommended way to provide an unlicensed user access to a resource is to grant anonymous read-only/browse access to the project associated with that resource. I was able to find a comment in this feature request https://jira.atlassian.com/browse/JSDSERVER-4151 in which a user reported being able to configure legacy application link options by appending ?legacyEdit=true to the application link configuration page. Since the legacy authentication options are deprecated, your mileage may vary on whether this works or will work in the future though.