It is ridiculously simple too - just set threads to no longer accept comments when they get to however old MNHQ decides.
If someone wants to update an old thread they have two choices - contact MNHQ and ask for it to be reopened for them or, link it in a new thread with a title saying there is an update.
There is NO need to leave threads active for decades, its reallllllly poor forum management practice and I absolutely do not understand why MN do it.