It shouldn't have any bad effect on your account but deleting your old posts might affect your forum ranks. I heard someone with full member rank deleted his few old posts and it ranked down.
That will happen if the user who deletes his posts old posts deletes so many of those that less than 120 posts remain. 1 post = 1 activity point and if someone who has 120 activity points and 200 posts and suddenly that person decides to delete 120 posts then he/she will have only 100 activity points which will de-rank that member. A full-member rank requires one to have at least 120 activity points.
I have heard this before that deleting too much posts can have you loss your current rank. But I'm unsure if it's totally true. I have never seen any person saying, I was a senior member but I'm now full member. Is it really possible? Is anywhere in the forum mentioned about this? Like an official post from admin or mod? Can you provide a link? I want to see. I thought activity was just like merits. You cannot loss them.
IRREVERSIBLE