Remove support for Drupal 9

Drupal core requires guzzle 7, which includes this method.
Unless you have a very compelling use case, this is "won't fix"

Well, that's correct as for Drupal 10. Drupal 9 requires Guzzle 6 or 7 (see https://www.drupal.org/node/3268032 → ), and the module still supports ^9 as defined in salesforce.info.yml.

Yes, Drupal 9 is EOL, but I think:
1. This fix should be included to keep supporting ^9
2. The salesforce module's major version number should be increased and drop support for ^9.

Ah, thank you for the additional info.

Dropping support for D9 makes sense to me.
I'm not interested in supporting any unsupported Drupal core versions.