In a move that's sure to delight many, Ubisoft's city-building game is a fan-favourite, and Steam achievements are undoubtedly a great addition to the release.
You should be able to unlock Anno 1800's Steam achievements retroactively, with the update history over on SteamDB indicating that achievements appear to have been added on April 12. The game's last update was on April 4, so it looks like these achievements tied to your Steam have been dropped in out of nowhere.
There are a grand total of 112 achievements in the Steam version of Anno 1800 now, and they range from having stocks of certain foods and resources to some more DLC-specific unlocks as well. So if you want an excuse to jump back in and see your Steam achievement list rocket up, there's no better time than right now.
Since coming back to Steam, management game.
I wouldn't start assuming that all of Ubisoft's Steam-migrated games will get achievements, though, as there's no indication from the publisher that this is the plan. While Steam achievements undoubtedly make the experience just that little bit better, being able to track progress toward certain external goals, it's not something I'd hold your breath over with Ubisoft games on the storefront overall.
If you are jumping back into the game with the advent of Steam achievements, our breakdown of all the Anno 1800 mods to spice up your save file as well.