As long you dont report threads and posts that are back in 2015 for spam something like a " Good Project " you can report them if you think its against the rules .
This back in 2015 is that an established border line we are looking at or you just made it up. However obviously there are no point to report such old posts.
I've just read a thread that was last replied on April 2019. I believe that need to be Moderated but it's almost dead. is it good to report such dead threads or simply ignore them?
In this case report to lock the topic will be more appropriate in my opinion if the thread is dead of course.
There is no time limit to what you can report.
That's what I thought too but seems like Lafu got a border line with a specific year 😁