That is completing actual tasks for the company. You are effectively acting as a consultant for them. And I have heard of a number of cases of people actually coming back to fix something briefly, which is even further in that direction.

Sure, coming back to fix something is absolutely completing actual tasks for the company. But I certainly disagree you can generalize answering a question as "effectively acting as a consultant for [the company]".

Someone who replaced you but you don't know reaches out asking for help on their work? Or you're being asked questions you'd need to dig in to? "Sorry, I don't work there anymore".

But answering a text message from someone you had a good relationship with, who is asking a question in good faith, the answer to which you can recall without effort? Wouldn't agree with calling that consulting.