Alright so here's what happened:

I picked up a copy of House of Lust from JAST, and guess what, the JAST version has always-online DRM as well.
When I did a file comparison of the game's exe in HxD (a hex editor that also has a few other features like byte by byte file comparison) , the GOG version is exactly the same version as the JAST version (the same comparison was done for the data.unity3d and shareedassets1.resource files, which also matched). When I did a check of both game's directories, all other files seemed to match as well (just by looking at the file sizes).

For the GOG and JAST versions, the game runs while online, and no longer functions while offline.

I sent JAST a ticket concerning the issue. I'll give a heads up if/when JAST gets back to me.

Edit:

This just in: the GOG team actually got back to me on the issue (after I replied back to the initial ticket email), and they were able to replicate the issue on their test machines. They are going to look into it now (no ETA).

Great news!

Tbh ... "no ETA" in GOG speak apparently means ... maybe we can fix it maybe not ... in any case we will never again reply to your ticket and we are glad that we can close it now.

True. But at least they replied once. Which is more than closing the ticket as 'solved' without even replying.
But until they actually do something, the game remains on the list.

Too bad that it isn't big enough to cause an outcry like Hitman did.

Well. it's been four years now since a blue wrote:
as long as these additional features and rewards do not affect the single-player offline experience in a major way, we believe that the developers and publishers should be free to design and sell their games in a way they choose.

Since then, they have apparently reduced that stance further to:
we believe that the developers and publishers should be free to design and sell their games in any way they choose.

GOG is, by now, a DRM agnostic store. Which is a business decision they are free to make. They just should stop pretending otherwise and start labeling their games according to DRM category. But they won't do that. They decided to rather lie and pretend that they are still DRM-free.

Well, actually I do wish GOG woul go back to actually being DRM-free. But becoming DRM-honest would be already an improvement to the state things are now.

It's a sorry state of affairs, for sure.

Big respect for keeping this thread maintained for all this time, dude. It's doing a really important job to keep up pressure on GOG to stick to their promises and be accountable.

This is a thread that deserves to be stickied!

Whilst I have bought some of the games on this list, this thread helped me make informed decisions beforehand. From an idealist perspective, I'm not OK with single-player content being DRM'd away since I'm never going to play the game online. I'd view the practice of DRM'd single-player content the same as pre-order exclusives: permanently denying me access to the full game.

However, I do draw a distinction between essential gameplay features and cosmetic fluff. In the case of the latter, if the game is good and the fluff is not something that would have mattered to me anyway, I'll still buy the game... grudgingly, and certainly nowhere near full price.

Another update concerning House of Lust:

JAST got back to me as well, and they said they would investigate the matter.
They also stated that they "definitely shouldn't have DRM on [their] store".

I'm glad to see the issue being addressed on both stores. I hope this leads to a patch/update making the game DRM free.