CIVICRM_MAILER_SPOOL has probably never worked for any type of outbound email except bulk mailings

I think the intention behind this constant you can set in civicrm.settings.php was to have all bulk mailings be logged into civicrm_mailing_spool and not actually get sent, e.g. for debugging. But it doesn't work for any other type of outbound mail because there is no associated job, and both the table structure and the "sending" function require a valid job id.

It would be useful to have this also work for other emails. There is CIVICRM_MAIL_LOG to write to a file, but the option to write to db is useful too (see also CRM-11190).