They are mostly obsolete. They might be useful for low-volume, low-priority messages or hobbies, but delivery is inconsistent. Sometimes they take a while to deliver, they often get dropped or throttled.
I worked with an app that sent weather alerts to subscribers and had used these gateways. It wasn't very high volume, only sent messages during active storms in the subscriber's area, and not more than one every 10 minutes or so. We hardly ever sent more than 100 messages in an hour for all subscribers, even during a heavy storm period. Still we encouraged users to use a push messaging app like pushover which we integrated with, since sms through smtp often didn't deliver or took hours to arrive. We considered setting up a real sms service like twilio but the push messaging apps had better features than sms anyway, so the sms over smtp option was just a hack for users that really wanted sms.
In 2011 they worked pretty well, but by 2018 they were frequently down or didn't work properly. Pretty sure that messages sent through them are much more likely to hit spam filters or get rejected for whatever reason.
This isn't a full answer, but I know of a few "smart" appliances out there that use these gateways to deliver messages. The washing machines in my college apartment (which were otherwise extremely dumb) did for some reason.
The amount of traffic that goes through these gateways probably isn't worth charging for, given the amount of headache it would cause for incidental uses like the above. But it would probably cut down on gateway spam, which seems to have increased lately (at least on AT&T).
That's a major difference between the US and the rest of the world.
In the US the recepient of a mobile call and obviously also an SMS pays. Either per use or as part of a package.
In the rest of the world the recepient pays absolutely nothing, the orginator pays either per use or by package.
So such gateways make sense for US-providers. But no sense in the rest of the world. Very few did always exist during the last 20 years even in the rest of the world. I tried to use them years ago and failure rate was something like 90% or more. If something is too good to be true it's typically not true.
Off the top of my head, Tracfone still operates like this in the US. As of 2018, they were selling service in "minutes", where one minute is also equivalent to 3 SMS or 1KB of data. The most common denomination is a 60 minute card for $20 (with a 90 day expiration), but in practice, many subscribers have a doubled minutes benefit for everything they add. They also have "just texts" and "just data" cards which offer better rates for those categories of service.
Tracfone targets themselves very narrowly at the "elderly and only want to pay for what I'll use/just want an emergency phone" demographic. If the subscriber is going to use the phone approximately at all, those are garbage rates, but if you only want to keep the lights on, $20/3 per month is about as cheap as it gets.
