Thanks to @misc , we have applied a workaround that works for now (but doesn't seem to work with latest 6.6.1 release from 6.6 branch, so we downgraded to previous release for now)

Keeping this ticket open so that we can all work on a different approach for auth as openid plugin itself is now deprecated (probably reason why it doesn't work on actual/latest wordpress release)

@misc : don't know if you had time to investigate this but I just gave it a try with https://wordpress.org/plugins/daggerhart-openid-connect-generic

While configured in wordpress, and also ipsilon, when it tries to reach out to ipsilon , it seems it's trying to use something that ipsilon doesn't support for openidc :

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/cherrypy/_cprequest.py", line 638, in respond
    self._do_respond(path_info)
  File "/usr/lib/python3.6/site-packages/cherrypy/_cprequest.py", line 697, in _do_respond
    response.body = self.handler()
  File "/usr/lib/python3.6/site-packages/cherrypy/lib/encoding.py", line 219, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/cherrypy/_cpdispatch.py", line 54, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python3.6/site-packages/ipsilon/providers/openidc/auth.py", line 825, in __call__
    return super(OpenIDC, self).__call__(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/ipsilon/util/page.py", line 85, in __call__
    return op(*args, **kwargs).encode('utf-8')
  File "/usr/lib/python3.6/site-packages/ipsilon/providers/common.py", line 105, in root
    return op(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/ipsilon/providers/common.py", line 91, in GET
    raise cherrypy.HTTPError(501)
cherrypy._cperror.HTTPError: (501, None)

openidc plugin now works on https://blog.stg.centos.org
Only things to know, when configuring the plugin against ipsilon idp :
Parse idp https://id.stg.centos.org/idp/openidc/.well-known/openid-configuration for parameters and so configure it like that at wordpress counter side (openidc plugin)

Login Endpoint : https://url-to-ipsilon/idp/openidc/Authorization
Userinfo Endpoint URL : https://url-to-ipsilon/idp/openidc/Userinfo
Token Validation Endpoint URL : https://url-to-ipsilon/idp/openidc/Token
Identity Key: nickname
Nickname Key : nickname
Email Formatting: {email}
Display Name Formatting: {name}
Identify with User Name : checked box
Link existing users : checked box
create user if does not exist : checked box

@misc : FWIW, that plugin works fine with wordpress current (6.6.2) so I also migrated https://blog.centos.org to openidc and wordpress and auth is still working fine. I also disabled/removed the older/legacy openid plugin (and fedora autologin that was capturing the login form to inject openid/idp url)

Closing now