Having some kind of text badge/warning icon if a page has set the "isAccessibleForFree": "False" flag set in its structured data would already help a lot.
Of course, not every site owners follows this, but for the site the original poster named as example it would work, for example (and the usual tools for coping with such sites such as lowering/blocking would not work here instead, as the site has a lot of high quality articles and only some of them are behind a paywall, basically the structured data is the only indicator you have here if something is gonna be paywalled)
And also, being only some kind of visible flag, every Kagi user can decide by himself then whether to still visit this as he should probably know if he has a subscription for that site or not.
I just checked a couple of sites (heise.de, spiegel.de, washingonpost.com, nytimes.com) and they all had that flag - seems Google now requires it to not give out cloaking penalties. Only thing I noticed is some people use true "JSON" false, while other sites use "False" as a string.