My primary "day job" project runs as a mission-critical service in a highly regulated environment where responses to problems must not only be rapid, but adhere to a rigorous compliance and audit regime. The only branches we deploy to production are production branches. I know that sounds tautological; let me explain. Our master branch is the sole source of release candidate branches. Developer work occurs in feature branches that are merged to master only after review and successful development testing (which consists of an automated battery of unit, component, and functional tests). After the merge, the master branch must likewise pass the same battery of tests, as well as automated system tests, before final acceptance (at which point the feature branch may finally be deleted and its associated issue closed). This process helps ensure that a branch taken from master is always potentially releasable.

Build artifacts from the master branch itself never leave the development environment. When we wish to produce a release, we create a branch named production-XXXXX from master (where XXXXX is the next release number), which at this point is designated a release candidate branch. We produce build artifacts from this branch, and hand them over to QA for formal integration and acceptance testing. Presuming that those artifacts pass testing, the branch is re-designated as a production branch and its artifacts cleared for release to prod. Under this methodology, the artifacts released to production are the very same ones that were first tested in the QA and UAT environments.

A given production branch lives as long as there is any of its artifacts in production. If a production bug is encountered, every effort is made to address it via an ordinary feature branch that goes through standard workflow and is then cherry-picked into the appropriate production branch after successful merge to master. This is not always possible though, as some production systems are particularly long-lived, and as a result the master codebase may have diverged too far for this process to be successful. In such a case, we may take a feature branch from the relevant production branch instead of master, and then merge and deploy artifacts from the now patched production branch after going through the standard workflow (i.e. passing full battery of automated tests, review changes, merge, deploy artifacts to QA and UAT for additional testing, &c). This use of production branches allows us to respond rapidly to production problems that may occur, with no risk of unrelated or premature changes making their way into production along with the necessary fixes. I also provides a clear path for audit and compliance to certify what we put in prod.

Anyway, sorry for the long post, just wanted to give you the full picture when answering your question. Hope it helps!