Achow101, thank you very much for your very valuable explanations on the topic!
My main goal in running Bitcoin Core full node is to help and support as much as I can the wonderful great Bitcoin network and community. It is great that even without accepting incoming connections one can still support the network!
Yet, personally, I have a pretty fast wide Internet channel and 24/7 server which I would very much like to do the maximum benefit for the Bitcoin network.
So, I prefer to stay with 28.1 version for now.
Maybe in the next versions the Bitcoin Core developers will find a solution to return the UPnP functionality (without issues).
Then I'll immediately upgrade to that new version.