The style makes no difference. You can be in the best MT school in the world, but if they don't teach you self defense, then whatever you learn isn't going to help you.
That said, the style has evolved into a sport as you noted. Much of what is practiced today will only go so far. The problem with niche sports today is that they are not well-rounded, that by definition makes them niche. That means, a typical competitor will be like the proverbial hammer as the only tool in the carpenter's toolbox, and what happens is that all of the carpenter's problems will be handled as if they were nails.
The way to tell if your school teaches self defense is to just ask them. Some will be upfront and say "no, we teach for the ring, not the street". Others will say "yes", and then let you figure out for yourself whether what you're learning is helpful on the street.
Some things I would look for are: are you being taught weapons use and disarm? Multiple foes? Multiple friends? Striking (including hands, elbows, head, knees, feet) and grappling (including locks, bars, chokes, trips, sweeps, throws)? Are you taught how to fall? How to get up? Are you taught to use your environment? Do you get to practice in different environments - asphalt, grass, etc? Do you practice in street clothes - including shoes? What about non-fighting techniques, such as verbal de-escalation, and changes in lifestyle? So, like, walking alone, driving, being in a bar, or being at a party; your choice of clothes and shoes; how to use everyday items as weapons; keeping your home safe. What about using a hand gun?
Do you even get a primer on what self-defense is? Many people have this idea that self-defense is all about a mugging. Any date rape victim can easily set the record straight. What will you do in defense of others - like your spouse or child?
The point is, things you learn to use in the ring are only a subset of what real self-defense is all about. It's not just about a mugging, or a one-on-one thing. Sometimes, it's about looking for escape routes, or talking your way out. Sure, there are overlaps. But self-defense is self-defense, and sport is sport. They are not the same. A hammer should only be used for some circumstances.
So it doesn't matter what Muay Thai used to be about. What matters is what your instructor will teach you today.