The first game to get such treatment, Crystal Caves, contains the original version as well. The second game, Secret Agent, does not. I would err on the side of caution and not trust that the original versions will definitely be included.

Apparently Sam and Max remaster or whatever has it too ..Atleast what I hear.. unsure if Bloodrayne remaster does but it seems to be from what others say too ....See I don't own the Sam and Max remaster as the devs lied about changing things and lines.. so will only get it when dirt cheap ..well as the Bloodrayne games had the remasters show up in my library as a freebie seeing as I already owned the O.G ones

Nice find. I wish I was there 7 years ago, the Anthology was a great value pack. DRM free from what I see.

Maybe someone should contact the Devs to point out the "missing" legacy versions and encourage them to treat future remasters the same way. I would do it by myself, but neither own Crystal Caves nor Secret Agent.

I wrote to them about it 5 days ago, and haven't received a reply.

It's funny that their web page doesn't even have Secret Agent
http://www.emberheartgames.com/

They might be more active on social media channels. So if anyone uses Twitter, Discord or Instagram, he could contact them there.

I know. It's awful. Remasters are okay, but why get rid of the original? Or maybe bundle the original with the remaster and give existing owners the remastered version as well. No actual reason for it.

Will this remaster even be compatible with RoTT Extreme?

Ok. I don't think this is exactly the thread for it but Sin Slayers was yoinked off of Steam so now for PC, it's a GOG exclusive. However, I doubt the game will be removed on GOG as all of the publisher's games were BANNED by Steam, not taken down by the developer/publisher themselves.

This issue also affects the game Against the Moon. The publisher was banned in February from Steam, returned in June, and got banned again shortly after, no explanations by any party. I'm not sure that they are at risk at GOG per se, but if you don't want to take any chances, by all means pick them up.

Nobody knows why they were banned? Like, for generating Steam keys for themselves and then selling them on their own on G2A and co? Afaik that is one reason some devs/pubs got the boot by Steam.

OpenTTD since they didn't include the source code in the downloads

Why should this indicate a risk of removal?

As long as GOG provides a method to let you get sources code of OpenTTD, it is legal.
GNU GPL does not force anyone to pack the source code with binary.
If you just want binary, it's fine.
It's only illegal when GOG refuse to provide source code of OpenTTD after you asked.

UPDATE
Several years ago, GOG put source code of ScummVM in some game installers.
It made those installers bloat, and the effort is entirely unnecessary.
No, you just do not need to put binary and source code together.

Are there any common traits or patterns in the non-relisted delisted games? I hypothesized that games that haven't seen any major price discounts or increases are also at risk because it gives a very rough indicator of whether the gamedevs / publishers are still actively communicating with GOG. But it turns out only a small handful followed this pattern. For example, Chantelise, Chronology, Slender: The Arrival (not an exhaustive list). And even that's not completely accurate either. For example, Pier Solar hasn't seen a discount since Sept 2018).

Would be nice to develop some KPIs so we can anticipate them and buy them during sales instead of a couple days before their delisting is announced at regular price.

Aquaria might be at risk of getting delisted.

From what I could find, the game was made by 2 developers, and the main developer of the 2 suicided after he was accused of something bad on Twitter and everyone turned against him almost immediately. (not entering in details to not derail the topic, but if someone wants to look for more context, this guy is also one of Night in the Woods' developers, Alec Holowka)

After that, the game started getting less and less sales, and now it's been 1 year and 1 month without any sales, at least according to ITAD.

So if the lack of sales is a sign of lack of interest from the remaining dev, he might end removing the game altogether.

And on a side note, Aquaria's engine is open source and has an updated fork on Github. I don't know how much at risk either versions of the engine are, but I think it's worth keeping an eye on them.