But why can I still open this account if its deleted? And can I ask the reason why my account got deleted?
You probably created new one once your old account has been deleted in 2019 because all inactive accounts have been deleted if owner of the account hasn't logged in for a long time.
I searched the name that you mentioned "Ace-Khun" to check if it's still there but it seems like it's not there since I couldn't find it in "members section".
Most of the player's forum that was inactive around 2018 was deleted permanently. I myself got my forum deleted due to long inactivity.
When you came back in forum, did you have to create a new account or just log in with your account?
And I think your forum clearly shows that it was created in 2019, maybe you created it that time and forget about it as there are some new commands regarding creating and linking forum with in game account. As you said that your first forum account was created in 2015, then your posts and the date of creation of your account must still be there. But clearly not.
Like I said I never "created" a new one because I did not know it was deleted in the first place, I just entered the same username and password as the one that got deleted. Regarding to my posts, are you saying I'm lying about having an old forum account?