Any modern IDE will let you immediately bring up the subclasses with a single hotkey. If you have an abstract class with only a single subclass and that's not because new code is going to be added soon then yes, it's a bad design decision. Fortunately, also easy to fix with good IDEs.
In my last project every class had a corresponding abstract class, and then we used DI to use the real class. Good to be rid of it.