Heyo!
So, recently I've seen a lot of necro-posting and even accidentally did it myself(oops!). What I suggest is to implement an automatic message that will pop-up on the screen once you try to send a message on a thread older then 3 weeks, and asking you if you want to continue sending the message or not. I think this should be implemented over other suggestions, such as locking threads over 3 weeks old because there are some special situations where I myself, think necro posting should be allowed.
So, recently I've seen a lot of necro-posting and even accidentally did it myself(oops!). What I suggest is to implement an automatic message that will pop-up on the screen once you try to send a message on a thread older then 3 weeks, and asking you if you want to continue sending the message or not. I think this should be implemented over other suggestions, such as locking threads over 3 weeks old because there are some special situations where I myself, think necro posting should be allowed.