Thank you for sharing. Based on that description I can see why you doubt its usefulness. I've been a part of retros that cover the whole spectrum of usefulness. Some feel like going through the motions just so we can check a box whereas as others have helped transform our team dynamics.
Did you you always a scrum master running the retro?
This is definitely a challenge. You can spend an hour coming up with great ideas for how to improve and then forget all about them as soon as you start the next project. I've seen that happen a lot.
What does a single notification platform look like to you? Is it a web, desktop or mobile app? Would you use a separate application just for keeping track of your notifications? I can see how it would be useful but also it seems like it could be a pain to add another app you have to give attention to.