In some cases yes. In others they were just used for energy to turn a flywheel for sawing wood and no electricity was involved. For the electric ones they may have been able to generate 200MW at their peek just after they were built. But after a hundred years of sediment they may peak out at 50. If the cost of fixing is more than it would generate in say (pick a number) 10 years they may decom it. Which is its own set of challenges. Most of the documentaries I see start off with 'this dam has been here since 1930 and the company went out of business in the 50s'. As most of the reason for the smaller dams to exist have been superseded by better tech. These things are typically 5-10 ft high. They are not much. But there are thousands of them.

Also many of these things are not 'automatic' they need care. Such as switching things at the right time, replacing parts, etc. You may not even be able to buy the correct switches or motors anymore (the NY subway has this issue) and maybe you can get one used from some other dam that is being decommissioned. So they just turn the thing off and walk away. Usually they cut some deal with the local gov to take control. Who think they are getting a 'free park' when they are getting a giant mess. Sometimes they even say 'you do not want it' and the local gov is dead set on getting it for a park. So you end up with things like love canal.