My approach has been to use Keycloak as an identity broker. It's implementation is quite robust and supports a lot of flexibility in terms of mapping custom assertions and the like. But the actual application "only speaks OIDC" and relies on access tokens to be reissued by Keycloak.
The way it works in enterprise is that somebody wrote guidelines years ago that external software must support SSO with SAML. Then the guidelines were never updated and in some cases the company never realized they can support OIDC out of the box.
The exception is education, where Shibboleth is very entrenched with federations spanning all universities in some countries. Another exception for healthcare/defense that may not have updated any of their systems for 20 years, though they may not be customer if they have no internet connectivity and no SSO :D