Ah - but not on the system posts it seems!!!
I updated the FAQs, made a comment, and then wanted to set it to self-destruct after a month. But the option isn’t there:
I then checked it here on meta.discourse.org (where I am of the lucky TL4s), and didn’t see it either. And jumped to a (faulty) conclusion!
Ok, now I know that it isn’t available as an option on system Topics for some reason. Weird!