This is almost more like a feedback than a feature suggestion (yet it's not a bug report): today I ran into a probably rare situation where I had outposts that had not been converted into cities yet despite being into the contemporary era (in order to stay under the limit). Once I unlocked one of the latest tech increasing that limit, I went for the "evolve" button only to discover an "action prohibited" prompt. I looked everywhere in the game, and couldn't understand what was the problem. I looked it up online but the key words only gave me forum discussions of people who didn't realized picking Huns and Mongols cultures came with specific game mechanism for outposts and cities, so nothing to do with my own problem. It took me a few minute to remind about the settler unit and after a few turns of creating it and moving it onto my outpost, I had the confirmation this was the solution to my problem.
To be honest this was so non-intuitive that even now I'm only 95% sure that's how the game is supposed to work (completely replacing the standard city creation mechanic with the settler unit special capacity). So if that change of mechanic is completely intentional, I think it would be a good thing to make it way more clearer by, at the very least, replacing that "prohibited action" with "use a settler to convert that outpost into a new city" or something like that. And there's probably even better ways to do that.

Were you playing as the Singaporeans?  Because like the Huns and the Mongols, the Singaporeans are also unable to found new cities from outposts (in fact, using a Settler to get around this limitation is something I've never thought of and is an interesting way to bypass it).  Otherwise, the mechanic for turnout outposts into cities shouldn't change; it's still there for the other (non-Singaporean) Contemporary-Era cultures.

Aaaah so it WAS linked to the culture I had picked. Indeed, it was the Singaporean. But I never found indication in their description that they prevented the creation of new cities. In fact it must only be true beyond one created city since I created, well, Singapore itself ^^'... So maybe this limitation should be made clearer and it should also made be clear if my settler technique is "legal" or not. If not, maybe just remove the ability to create the unit (not a problem for the rest of the game since it's the last era) and if it's meant to be, maybe specify in the description that this is still an option.

I need to correct my previous declaration: I have just found where it is said that outposts can no longer be evolved into cities for this Era. I still think it should be made clearer (you have to hover the era with your cursor in the culture/fame screen) but at least it is said somewhere.