The 2018 publishing date was the initial version of the game. Itch.io unfortunately doesn't list the last time the game was updated, which confused me at first as well. However, most devs here upload patch notes whenever they update. Those patch notes are dated, so you can usually check if a game is still in development simply by checking the patch notes.
I was unaware of that. It still makes no sense as to why there is no "Last update: " line for the last time you updated the files in the download. Maybe this is something that we could potentially petition to have implemented? It would be nice to be able to see the date of the latest update in the "more information" button.