SpringBoot Update documentation on using H2C to consider running behind a proxy that's performing TLS termination

Comment From: wilkinsona

When this issue was originally opened there was a note that advised against using h2c:

Spring Boot does not advise using h2c, the cleartext version of the HTTP/2 protocol. As a result, the following sections require you to configure SSL first. If you still choose to use h2c, you can check the dedicated section.

This was worded too strongly and was removed in https://github.com/spring-projects/spring-boot/commit/713c0fce7c7c836505714052221d36014a69411b. With that change, there's not much left to do in this issue other than mentioning a TLS-terminating proxy server as being a reason for an app to use h2c.