This addon will display a warning when trying to reply to old threads.
Here.
xenforo.com
Here.

Unmaintained - TPU: Show Warning when bumping Old Threads
This addon will display a warning when trying to reply to old threads. The warning can be be bypassed by checking a checkbox. No database changes, threads will not be set to "locked". The age cutoff defaults to 1 year, but can be adjusted via...
