Has this been abandoned by the developers?

Atleast on GOG it seems.

Steam customers are also waiting for updates, but devs are still active on Steam forums. They just left GoG, and I think one of the reason is the Steam workshop implementation they just made.

This is really startying to be a recurrent scheme :
-a small dev team needs visibility and money to have a good start, so they take it whatever they can (including gog)
-the start is good and they proceed fast
-the biggest customer group (steam) ask for workshop
-devs please the biggest customer group, so gog version of this same game is stuck at some pre-steam-workshop-state for good.
-devs, no they are big enough, no longer needs the gog customer niche, so they just abandon them to an unfinished version, or they refund them and cancel the game of gog catalogue...

Devs are sometimes concidering GoG as some free kickstarter in my humble opinion. (even if they don't think about leaving gog before deciding to take the Steam workshop path)

Finally devs answered me about update plan on GoG, and about the Steam workshop path.

"Hey! yes we have developed the Super Punk factory - which is essentially a cut down version of workshop - for this very purpose. we'll roll this out on gog later this year" [SuperPunkGames]

https://steamcommunity.com/app/489460/discussions/0/2650805212048089219/?tscn=1525851021#c1694921076568776402

I also have invited them to come here to keep us informed with their own voice and words.

Crimson Zeta (still Steam forums) 16 juil. à 12h05

"Both the GOG and Steam versions of TLL should have recieved the same updates - minus the GOG version's access to a form of Steam Workshop, of course.

A GOG workshop/factory is in the process of being built and will be implemented later this year as we ramp up development again for TLL.

We apologise for not maintaining regular communication with our GOG players, and will certainly be trying to improve this!"

I saw this on sale and thought I'd give it a try but after reading this thread...No.

The GoG version of the Last Leviathan is currently up to date with the steam version. The recent update brought flying blocks to the game.

An update thread was made not that long ago:
https://www.gog.com/forum/last_leviathan_the/fly_you_fools_update