Classy cannot be extended in Drupal 9

Problem/Motivation

Steps to reproduce

As a user, I should be able to visit a page that makes use of breadcrumb navigation. The template file block/block--breadcrumb.html.twig attempts to extend the classy theme, which apparently is no longer allowed (https://www.drupal.org/node/3103178).

Proposed resolution

Maybe there's a more elegant solution, but I was able to just use the needed extended classy template (block/block--system-menu-block.html.twig) as block--breadcrumb.html.twig and adding the original block--breadcrumb content within the block--system-menu-block block content. If this seems reasonable I will post a patch.