See MDL-38081 for context.

The file blocks/login/blocks_login.php uses a .btn class on the div round a button.

This issue is about removing/renaming it and any dependencies.

A few themes (Canvas, Formal White, Form Factor) use this class to add padding or margin above/below the button and Standard Theme uses it to center the button with text-align center.

The simplest and least invasive change would be to give it a more specific name e.g. .block-login-btn

(I also found a usage of .btn class in some question renderers, but in that case it's actually on the input itself same as Bootstrap. Still hardcoded, which is bad, but not impacting use of Bootstrap)