I downvoted because "it's normal for people to do it" but it is not appropriate for the supervisor to expect the asker to do it. Such work is voluntary, not slavery.

@AnonymousPhysicist Oh I totally agree, it's voluntary not slavery. And that's what my answer says, while making some points for both sides (why it's good to volunteer, and why one isn't compelled to)

Then I think you should edit your answer to be much more explicit, more on topic, and also much shorter. The question is about supervisor's expectations and asking for pay. It is not about what the asker should be willing to do.

@AnonymousPhysicist Maybe you should write your own answer then.

@SSimon It's normal for academics to continue working on papers they contributed to in undergrad when they start grad school, papers they worked on in grad school when they start a post doc, papers they worked on as a post doc when they get a job as a professor. The research is theirs, not their advisors. It's normal to collaborate with your past advisors to complete projects. They don't usually pay you when that happens, you're getting paid for your new position. Lots of academics at non "R1" schools also do their research entirely "for free" while being primarily paid to teach.

It doesn't make it acceptable. It is just exploratory. OP works for industry

@SSimon Did you read my answer or just the bold? The whole point I am making is that expectations and practices are different in industry and academia. It's up to OP to some extent to determine what makes sense for them. And the word you want is probably "exploitative" rather than "exploratory". I think that depends on who you think is doing the exploiting. You seem to think of academia as run like a bunch of fiefdoms owned by PIs and that researchers are mere employees. Not everyone sees it that way. For one, many of the people working in academia are taking a big pay cut to do so...

...that suggests that they see some other intrinsic benefit to being in academia: whether it's the privilege of advancing knowledge, a specific goal towards helping some part of society, or something else. Not everything is about money. Being forced to work for free by threat of some sort of punishment is a problem, but working to get your personal research published isn't necessarily.

It's even worse, they are treated most of times as slaves or cheap labor

@SSimon Not in my experience, I'm sorry if that's yours.

@BryanKrause Anyone is allowed to critique an answer without writing their own.

@user76284 Critique, yes. But there was a directive to significantly edit, that's beyond a critique.

Some parts of this answer are really bad. 1. Making unreasonable demands on someone's time is not a matter of perspective, and should certainly not be justified as thinking like an academic. 2. OP does not owe their former adviser anything, and certainly does not need to compromise with them. They need to decide whether they want to invest time, not please their adviser. 3. Academics don't work for free, but are highly paid professionals whose jobs entails collaborating with researchers outside their institution. 4. Idem for "basic authorship duties", those are part of working in academia.

"In academia, it is normal to "work for free" a bit; in industry it is not" It depends where, in a lot of places (especially the kind one has in mind when comparing to academia) it is very much normal to "work a bit for free".

@SSimon: "No Bryan, it is not normal to work for free after graduation in academia". Really? That's odd, because I personally don't know a single person who has graduated with a PhD and hasn't done some finishing work for "free". Quotes around free, because that work had benefits for the graduate that were not direct monetary compensation.

Well that doesn't make it fair or ethical

@SSimon It only makes it fair or unethical if you insist on thinking of academic effort as work you do for a supervisor rather than work you do for yourself under guidance and advice of a supervisor. I have a big problem with the first way of thinking about things because even though I think you're trying to defend the junior person here, in doing so you're giving all the ownership to the senior one, and making it their project that the junior person works on, rather than the junior person's project that the senior advises.

@SSimon: it's also worth noting that in many cases, this agreement for "free" work was directly to help the graduate. For example, suppose a postdoc opens up that the graduate would like to take. Then an advisor may make the agreement to have the student graduate earlier with the understanding they would be involved in publishing their graduate work. This two-fold helps the graduate: they bend the rules to get the postdoc and get their publications. With that said, in some cases advisors may unfairly ask for post grad work, but in many cases this is specifically to help the student.

It's impossible to judge the concrete situation from the internet, but you could turn it around and say OP gets supervision "for free" while completing essential final steps on finishing a paper. As with most things, they can take it or leave it, even more so that they are no longer financially dependent on their supervisor...