Checking archive_mode on pgBackRest

Stefan Fercot shares some advice:

A recent question on the community channels described a difficult situation: a standby had been promoted with archive_mode=off, and restarting the new primary was something the team wanted to avoid. Could they enable archiving on a downstream standby and take a pgBackRest backup from there instead?

Their attempt failed with archive_mode must be enabled, even with archive-mode-check=n. Removing the checks from pgBackRest’s source code allowed their proof of concept to succeed, but was that enough to trust the approach?

My recommendation was to return to a supported configuration, either through a restart or a controlled switchover. But I wanted to take a closer look and see whether archive-mode-check=n might allow the standby-based approach.

Read on for that answer, as well as some of the issues you might run into.