I have rechecked this. It appears that pg_createsubscriber writes the recovery configuration to the output and only in verbose mode. So, it's far no guaranteed that this information would be accessible. One may run pg_createsubscriber not in verbose mode or don't save its output. I suggest we should re-implement this in a way Michael proposed [1]: save the configuration to pg_createsubscriber.conf.old file.
Alexander, I'm not in favor of saving additional files. This approach seems to replace one type of leftover artifact (recovery params) with another (debug-files). Neither option is good.
As Michael pointed out, the parameters might be useful for post-debugging purposes. This suggests to me that they are, by nature, debugging information. Therefore, it seems appropriate that they should be captured by the verbose/debug mode. If verbose mode isn't used, we lose more than just the recovery parameters — we also lose the sequence of commands for managing replication slots and other steps. Following this logic, why not save all information in non-verbose mode that might be used for debugging?
Robert, I'll think more about a valid scenario (including the one you proposed) and get back with results later.
