This was used as a failover mechanism - if a control doesn't specifiy its own i18nBasename, we basically fall back to messages_templating_custom then to messages_templating.
Given the current bundling, it seems quite irrelevant to still use this.
messages_templating's keys are currently only used in samples. Only a few are still in use for the new samples for 4.0 (
MAGNOLIA-2506), namely those for the "dev-show" dialog. Let's move these to the samples module and the appropriate bundle.