IIRC there’s either no limit (or a very long limit) to the ‘subject’ field specified in the relevant RFC for ‘emails’, so you could (probably) put your entire message in the subject!
Oh it’s possible. My uncle does this and it drives me nuts!
Could it have to do with the increase in popularity of more inmediate communication formats in the business environment such as Slack?
Where email is then reserved for longer content and external clients, instead of “chatting” over email with a couple lines per message.