The answer obviously depends on the project and the best course of action can only be found by talking to the core devs / maintainers / community around the project.
There is two ways to approach this: You can (start to) go through issues and make appropriate comments, maybe with links to where / when it was implemented. Concurrently (or before) you may contact the maintainers (is there a discord / irc / e-mail / dedicated forums ... way? Use that to establish direct contact, if possible) and ask about the best way to do so. They even might give you some priviliges on the repo or issue tracker so that you can actually close issues or flag them appropriately (maybe they would want to review some decisions).
Being involved in a similar project, I shall be delighted to have such help offered. Personally I'd tell such person to go ahead and comment on the issues. And when I see that it is substantiated with a considerate eye, I shall be quick to give that person the necessary power and tools to do that properly.