Update on my previous post: in my case, although I was not seeing any output from Drush that the 8003 update hook has run, it appears to be doing so. The cause of the code error is more likely that my site has a copy of the stage_file_proxy.yml in config/development, the contents of which override anything else in a development context. Ensuring the additional settings are in that copy of the file eliminated my problem.
FWIW I hit this problem after updating from 2.1 to 3.0@alpha. As far as I can see the stage_file_proxy_update_8003() did not get to add the proxy_headers into the config. However I have not reproduced this in isolation, I am updating code from 10.1 to 10.3 and hitting numerous issues.
I am going with #56 which appears to work for my requirements (replacing a basic country selection with an Address field that only captures Country & Administrative Area). Not sure why anything other than "- Select -" would be a better placeholder so not applying #57 myself.
Thanks for the work!
I am adopting the patch from #8 as a much-needed solution for the rendering limit and general performance issues. I agree with the suggested refinements but for now this makes it possible for me to provide the site functionality.
I think this change is fairly straightforward and will be useful to other users of Easy Breadcrumb. This is the first contribution I've made to the Drupal 9 world so I hope I've got it right.