We're using the redmine_openid_provider plugin for many years on 3.x and want to upgrade to redmine 4.x but cannot due to the lack of a 4.x version of the plugin.
Given the miniature size of the plugin, this is probably a trivial task for anyone who knows what they're doing.
At osmocom we are a open source developer community, but just not with ruby/rails knowledge :(
See https://github.com/osmocom/redmine_openid_provider for pluging code
See https://osmocom.org/issues/5084 for a description of failed attempts so far.
I would like to assist you with your requirement, please reach me at seth (@) cisinlabs (dot) com to discuss further.