IIUC, Previews features are meant for Android-only, but they are currently tested on Linux because there are no Android browser_tests (see issue 611756).  They can also be enabled as a base::Feature on Linux to allow manual testing for development/debugging.  See:
https://cs.chromium.org/chromium/src/components/previews/core/previews_features.cc?rcl=cb343f3c5acf70010afafa8c498ff5104a533493&l=15

However, there's an unnecessary risk of accidentally enabling these features on desktop via field trial configs (which has happened to other features in the past).  I'm concerned about this for Previews, since there are security risks for some of the features if they became enabled on desktop.  For example, content scripts might inject into Lite pages and run active script in a page that was meant to be passive by design, possibly violating some of the guarantees of that mode.

It would be better to have a guard to ensure that the kPreviews feature can't be enabled on desktop platforms via Finch, unless we're either inside a test or a developer runs with a command line flag to make it possible.

This could be done by setting a boolean in something like a EnableDesktopPreviewsForTesting method, and checking for that or a --enable-desktop-previews-for-testing flag within previews::params::ArePreviewsAllowed().