Hi folks, I'm pretty sure that I have seen explanation for that somewhere, but I cannot find that now. Is there a document describing reasons why Kratos does not support signing-in with OIDC (that gives us trusted/verified e-mail address) to an already verified account (that signed up with "normal" password but does not have the OIDC account linked)?

See here https://www.ory.sh/docs/kratos/social-signin/link-multiple-provider-account#automatic-account-linking

That's exactly it, thank you!