$ composer install […] Package guzzle/guzzle is abandoned, you should avoid using it. Use guzzlehttp/guzzle instead. […]
The following packages seem to use that (links to upstream pull request):
I'm currently regenerating a new composer.json for gerritdrost/phenum to see if that solves the issue.
Meanwhile, I'm preparing a pull request for oauth1-client.
Composer allows only one version of a software.
As they break ascending compatibility, they changed namespace name.
The OAuth library uses the old version, but the new package provides a new version.
They'll upgrade to the last guzzle library at the next version. Project doesn't maintain any release engineering calendar so this is "soon".
No really interesting to track.