I wrote a mod for this for 6.4.2...
https://www.ubbdev.com/forum/showflat.php?Number=112548 I have since stopped using it in favor of just locking threads that have not been active for 365 days.
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UPDATE w3t_Posts SET B_Status = "C", B_Kept = "K" WHERE ( UNIX_TIMESTAMP( ) - B_Last_Post ) / ( 3600 * 24 ) >= '365' AND B_Status != "C"
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I do this manually, but it could easily be setup as a cron task - I'm just too lazy.