As for the spyware accusation... from what I read, Take Two slapped their usual privacy policy on the game after they bought it. The community didn't take that nicely.
I don't remember ever seeing any evidence for spying beyond policy discussion and would be surprised if there is anything beyond common telemetry.
This is incidentally a very warm experience for me - I remember launching KSP some time after 2018-05-25 and being greeted with a request for consent for tracking. And the ability to say "no".
So it has neither been proven nor disproven that the game has privacy-invading telemetry but TT could add it in any day? Add to that that Steam forces you to update games if you wanna play them and that this probably affects all TT-owned games now.
Isn't that true of the vast majority of software these days (e.g., the vast majority of Steam games)?
(I mean, yes, that's of course bad, but it seems like a relevant distinction whether it's uniquely bad or just a non-noteworthy participant in a bad ecosystem.)
You should be fine with copying the ksp directory out of steams reach and run it from there, since the game doesn't appear to use Steam as copy protection. It certainly didn't when I was into modding years ago (still have quite a few older versions with different mods laying around) and even an up to date install still seems to start fine after exiting steam / doesn't contain any dlls looking like steamworks.
Just checked, the game runs fine without Steam. You can even launch KSP_x64.exe to skip the launcher, if you want.
The most interesting part of the log:
[ERR 10:20:15.181] [SteamManager]: SteamAPI_Init() failed. Refer to Valve's documentation or the comment above this line for more information.
[EXC 10:20:15.185] InvalidOperationException: Steamworks is not initialized.
Steamworks.InteropHelp.TestIfAvailableClient ()
Steamworks.SteamApps.GetAppInstallDir (AppId_t appID, System.String& pchFolder, UInt32 cchFolderBufferSize)
SteamManager.Awake ()
[LOG 10:20:15.218] Loading data opt-out preferences from PlayerPrefs
[LOG 10:20:15.244] Requesting data opt-out preferences from https://data-optout-service.uca.cloud.unity3d.com/[stack of unique parameters]
I always used to launch KSP via CKAN [0] so removing the ability to launch independently of Steam would have killed a lot of utility for me. (CKAN is/was the best mod manager for KSP, handling dependency resolution, config options, multiple mod profiles, compatibility checks, and even mod discovery)
It is available on GOG with downloadable installer and that version works fine on an offline Windows 10 system. I think I've heard that the Linux support is fairly good too but I haven't used it personally.
Unfortunately, it seems like essentially all games these days have at least basic phone home telemetry, even the DRM free stuff that still works if it can't connect. The only way to avoid is to block the game from accessing the network one way or anther. That being said, not everything has RedShell level stuff, although a bunch of games did and possibly others are using similar stuff that hasn't been identified. Take Two, like most companies caught with RedShell, solemnly promised to wait a while before reintroducing something like that and to do it less obviously next time.
IMO, it is best to assume that games are hostile from a security perspective. On Linux, at the very least run them under a different user without access to any important data. For Windows I'm not sure that will help much due to the permissions they require but I don't know Windows very well.
I don't remember ever seeing any evidence for spying beyond policy discussion and would be surprised if there is anything beyond common telemetry.