Question

Can we use SAML 2.0 without using ADFS?

Answer

Technically yes. 

K2 does support SAML 1.1 and SAML 2.0.

We also use other authentication providers that uses SAML 2.0 like Windows STS, etc.

However, due to lack of testing, it's not certain this will work with just any auth provider compatible with SAML 2.0, as we have had limitations in the past.