Since this is a patch on composer.json, I had to test it in a different way. ...

Since this is a patch on composer.json, I had to test it in a different way. Here's what I did.

1. Setup a new Drupal 8.9 site for testing
2. Added php-http/guzzle6-adapter with dependency set as 2.x-dev as 1.1. This ensures that we can install Geocoder module. Patch doesn't help here as patching happens after composer resolves dependencies. But we get the code from 2.x branch. It's important that we do this before adding geocoder as it gets tricky to update after that (but not impossible).
3. Install geocoder module as normal and one of the providers: composer require drupal/geocoder drupal/geofield geocoder-php/bing-maps-provider
4. Added a geofield to one of the content types and a text field to input the address
5. Double check the file at vendor/php-http/guzzle6-adapter to verify that the requirement for php-http/httplug is ^2.0. This is the actual package which holds the PSR-7 compatible ClientInterface. Also verified vendor/php-http/httplug/src/HttpClient.php for good measure.
6. Created a test node with a sample address and observed that the geofield was being set with correct lat/long coordinates.

I believe the above test is enough to mark this as RTBC. I should say that I was also involved in determining the dependencies with @yonas.legesse and why our code which needs a PSR-7 compatible client was not accepting the client available to Drupal. From the code differences between 1.1 and 2.0, there shouldn't be any break for geocoder.