Ah the joys of maintaining Open Source libraries :)
In this case, possibly caveat utilitor (user beware) applies, and provided their tests don't fail they can use a new version of the library. They chose to use your library after all, and there is no commercial contract keeping them there or forcing their use of a specific version (unlike many SaaS things with APIs!). Serious users may want to write some test cases for you, so you both know when their contract is broken, they may even like to fix that breakage? This leverages the value of open source to provide visibility and options for all parties.
Regarding the operational sampling / replay thing - we don't do this yet (I did say toying with the idea, not shipping :)), but we've been looking at putting what amounts to a transaction recorder in the sidecars that terminate TLS and manage request routing in our stack. The problems are less technical than legal/privacy for us, being a major processor of sensitive info. We already record API call failures into an incident log for investigation, giving us another option to build requests that exhibit the same failure but with synthetic data, that we can push back up the pipeline to open development areas.
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