They were not deleted on purpose. There was a technical error a while back and most old threads before a certain year were archived. We still have them, but since there were so many, moving them back is a long process. If there are any specific threads you are looking for, please let me know and I will go find them. Currently I believe just TLs and PCs can help with this.
Edit: Just checked and a TL or
PC can help. Emmie (Golfing girl) & Forums Officer can help as well. If you want all of your threads (instead of a specific one) I can search and have those brought back too. It actually helps when people make these requests because we can prioritize their return amongst so many others.