I can't find the word "multiplayer" in the review of Brummelbauch. Can you?

I also don’t see it mentioned in many of the reviews for Prison Architect. It's obvious that multiplayer mode requires an account—that makes sense. But I’m only interested in single-player anyway.

By the way, an official GOG team member has read this post, but the (possibly misleading?) reviews are still online.

It’s also questionable that no one is checking the reviews (and that potentially misleading reviews) have been left online for years. Above all, this can discourage potential buyers.

Edit: Can't find a button to report a review. Is this the reason why nobody report it? (Or no one reads the reviews after a buy, because after that, there is no reason for this)

Try reading the "Overall Most Helpful" review instead; the one at the very top (by bgilliford) that every user must pass in order to see the one you cite:
"The multiplayer aspect is completely locked to the Paradox multiplayer platform and is locked behind a paradox account login screen."

It's also not possible to edit a review once posted, so any outdated - and previously accurate - reviews made in good faith may mislead customers years down the line; after they have been rightly voted as highly helpful by contemporaneous users. But a lesson learned for you today, I suppose: GOG reviews aren't a reliable or accurate source of information (are they ever, anywhere?) Some times they are useful, and other times bogus. Try using the PCGamingwiki or requesting confirmation from users in the forums or something instead.

Then all's well that ends well! Have fun playing without DRM!

But on that note... what happens to Stellaris once Paradox no longer wishes to maintain its servers? Is it possible to play multiplayer without their servers, peer-to-peer or the like; or will it become a single-player doorstop when that happens?

While the report button still exists, it only appears after you have down-voted a review - but the button is no longer functional. To report a review, you would have to take a screenshot and keep a copy of the URL to the store page, as there is no way to link directly to individual reviews. Next, you will need to log in to the GOG Support Center, select Problem Type: Game Review, fill out the form, attach the screenshot, and explain why you’re reporting the review. Don't forget to add the link in your description! Alternatively, you could contact king_kunat, which is likely the faster option, if he is available, that is. Our dear king is quite busy, after all. :P

So, the reason hardly anyone is reporting reviews is that the process itself is neither straightforward nor quick.

The good news? I’ve heard they’re working on a new and improved review system. I'm hoping, that one of the improvements will let us report reviews directly.

Reviewers on GOG are some of the most illiterate people on the Internet, so take reviews with a grain of salt.

...Unless that user has set the sorting option to anything other than "Most helpful", in which case it's possible that the top-rated review will be several pages in, or even among the very last reviews (especially in my personal default view, "Order by: Most Recent").

I used and use default.

Whereas I prefer "Most recent", precisely because it's usually one of the earliest reviews that gets rated "overall most helpful", and this status tends to be reinforced over time for a variety of reasons. I'd rather gather more recent impressions -- especially since, as has been discussed, GOG has never allowed us to edit or delete our own reviews, so outdated information is very unlikely to be corrected or removed.

I have seen moderation notes/edits to some reviews to correct for bugs, but the thing is, this shouldn't be up to a moderator to fix themselves but more a poke given to users to edit their own reviews.

Of course, that would require such an ability.

The day Paradox pulls the plug, all DRMed features instantly die. From the top of my head, that would include multiplayer and achievements.

This is why DRM in multiplayer modes is not something we should tolerate.