The following are more natural expressions (I would say that yours are ungrammatical):
About that . . .
About that dessert you talked about the other day . . .
About what you talked about the other day . . .
I have a question about that.
I have a question about that car you talked about the other day.
I have a question about what you talked about the other day.
The issue is about that.
The issue is about that club you were at late last night.
The issue is (about) where you were late last night.
That can be used as a demonstrative pronoun to talk about something already mentioned or being currently "pointed to." (As with the first and second variations of the above examples—in the second of which it is followed by something else). Or, it should be replaced by another word. (As with the third variation of the above examples.)
Note that in the first example, the ellipses can be used to indicate a couple of things.
One is that there is a conversation taking place, and the person talking is actually trailing off and not saying anything else. In response, the other person might say, "Yes?"
The second is that there is actually something that follows but I've just omitted it for the sake of brevity:
About that: What did you mean?
About that dessert you talked about the other day. Did you finish it?
About what you talked about the other day—I still don't understand. [And so on.]