I saw a review dated Dec 7, 1922, saying that the game requires internet access, and doesn't launch when internet access is blocked by the firewall.
Today I installed the demo on Steam, and it launched fine. When I checked my firewall exception list, I saw that a rule granting the game access outbound internet connection was added (the culprit is Unity). I blocked the access, and the game still launched.

I'm not a Unity expert, but the game does what many other Unity gaes do, too: open a gateway for sending PC data to a Unity server. It seems to be a Unity compile default, but can be changed by the developers (some developers do it actually).
Perhaps the developer of Remains prevents Unity from creating this rule? It's illegal at least in the European Union, to collect data from user PCs without the user's approval (opt-in, not opt-out, byx the way).
Like I said above, many other Unity games do the same, and most players don't seem to care.

And back to the review I mentioned above: It's not a DRM issue, Remains i DRM-free indeed as far as I can say, but it acts as spyware, intentional from the dveloper's perspective or not.