While PKCE is not part of the OIDC spec (a similar protection is added by the state parameter), it becomes more frequent to have authorization servers that support it or even enforce its usage (it makes sense for an authorization server that also support standard OAuth2 clients).
Since adding PKCE is additive we could add it, authorization servers supporting it while do the verification others will do nothing.
https://tools.ietf.org/html/rfc7636