I respectfully disagree with you here, on the premise that they are intentionally scamming.
Being blunt here (I don't think this), it almost feels like you're purposefully looking for something to be upset with them over. But I'll give you the benefit of the doubt and assume that you're unintentionally looking at this in (what is in my opinion) the wrong way.
You cited a comment by them on the matter. I happen to accept their explanation. You say that someone trying to look into the game may mistakenly get the idea that they can buy then upgrade- and in fact I agree with this- however (as you cited), those offers are all marked with an expiration date. From a pure and simple business standpoint, their bases are covered. Nothing is hidden, and if this scenario were to happen it would (in my opinion) be the fault of the consumer, for not noticing this expiration date.
I do think something should be done, however I don't even remotely agree that this is an attempt by them to do any sort of scam. I think that perhaps this expiration date should be posted at the top. Instead of (I assume) at the bottom; or even worse, somewhere in the middle. Even better, perhaps they should lead these articles with a large "EXPIRED" at the top. Maybe they should delete the pages entirely, since it no longer serves them any purpose.
As far as the forums are concerned, with people talking about the possibility to upgrade, again I agree that this could absolutely be a source of confusion. I'm not sure if you were implying this, but I do not under any circumstances think that the company should be held responsible for people getting confused from forum posts made by random people. Not sure if you were implying that specifically, but if so I disagree. A potential solution, perhaps there should be an all-caps sticky saying that the promotional offer has expired.
Outside of the promotional offer, I also agree that there could be (an appropriate) assumption on the consumer's part that if they buy any of the older expansions, they would be able to upgrade. I don't think these should be removed, as again I accept their explanation, that if someone for some reason specifically wanted an older version of the game, they could get it. However, I do think it should be very obviously stated that upgrading to Universe is not possible. I also assume the same is true for upgrading from any earlier version to a newer one, being that you simply can't? I think this should be made abundantly clear.
So I'm going to quote you here to organize my thoughts and make sure I covered everything properly.
"Matrix Games / Slitherine is still, to this day, selling the original Distant Worlds (25.99€), RotS (16.99€), Legends (21.99€) and Shadows (21.99), without the possibility to upgrade to Distant Worlds Universe. The last upgrade was just a limited time "promotional" offer.
So to someone looking at the game and even searching for some information online (since the upgrade possibility is mentioned in forums and so on, the limited time is often not mentioned), it looks like you can buy the base game (eg if you cant spend a lot of money) and then upgrade later.
Or you have the first 3 expansions, were not paying attention to games during the "promotion" time or to matrix games in particular and now matrix/slitherine does not allow you to upgrade (DW:U was promised as an expansion to the DW community, not a limited time expansion/upgrade)."
Looks like I covered everything so far except for that last bit, about the promised upgrade. I'm starting to notice a general theme here with your complaints. While they are valid and logical, they are (in my opinion) too consumer-focused. Which is natural and I absolutely don't hold it against you for taking such stances. (By the way, just want to make it abundantly clear, I'm not being condescending just in case it might seem like I am) But man, there's a balance to be had here, an equilibrium between everything perfectly catering to the consumer and perfectly catering to the developer.
Right now I would say that everything you brought up just now, is absolutely too far slanted toward the "developer" side of things. He's selling the originals (which benefits consumers too), advertising for a better product on those pages, and there are no mentions that you can't just upgrade. It isn't obvious that each "expansion" is actually a standalone copy of the game, rather than- for example- how World of Warcraft's expansions work. I didn't see you offering any suggestions, but if you did I'd wager that they would be too far slanted toward the "consumer" side of things. What I have been suggesting is in the middle.
Anyway, moving on. As far as the promised upgrade. You complained that this emerged as a limited time upgrade.
"So you sell a product, implying an upgrade possibility, then make the "upgrade" only available for a limited time (since DW is still sold to this day, the "upgrade" time is even before the purchase time if you buy today), and then charge full price for the upgrade later on."
First of all (since I wasn't one of the original community), I'm confused if by "promised as an expansion" means given for free to those who already have everything, or if it just means that it'd be treated as a regular expansion, rather than being a limited time expansion... see that doesn't make sense, I'm going to assume that in every case you meant to use the word "upgrade" instead of "expansion".
"DW:U was promised as [a free upgrade] to the DW community, not a limited time [upgrade]"
So here I ask you. How else do you expect them to implement this promise? As you quoted, it seems they determined who was an original community member based on "details" they had... they emailed everyone offering the upgrade. Due to "logistical constraints" this had to be a limited time offer. So this sounds to me that everyone wasn't simply emailed a key for the expansion, they had to respond to the offer and would then have a key generated and sent to them. This is all I could imagine being the "logistical constraints". But again this is just another case where I take their word for it. They didn't explain it in detail, but they claim there were reasons for not being able to have it just always be available, and I believe them.
At the end of it all, I'd like to request that you try to relax from being so heavily on the consumer side of things. You seem quick to target companies and accuse them of evil doings when in reality, all of this is probably just an oversight on their part. It would be helpful if the developer or someone could chime in here, though I doubt that will happen. In any case, again I request that you be more open to giving people (and companies, which are people too) the benefit of the doubt.
Also, you mentioned that you had considered working on this again, but other than that you didn't really address anything I said... I'd enjoy hearing your thoughts.
Anyway, guess I'll stop myself here. I think I covered everything pretty thoroughly, and once again my post turned out way longer than I imagined... I'd also like to mention that this took quite a while to write, since I was constantly checking your post and doing my best to kinda fact check myself.