something they've started using in places here lately and seems okay as a plug and play solution is just swing gates that can easily be pushed open, that forces you to slow down but still allows most people to easily get through. It's not perfect since there's still some resistance and i can imagine some mobility aids making it annoying, plus there's risk of scratches and stuff.

But really i question the need for physical obstacles in the first place. Just put some thick layers of road paint to create a rumble strip and make it visually obvious that you should slow down.
And especially in this case (someone linked the location on gmaps) i just see no need for anything at all, it's already a bloody narrow passage that seems well used by pedestrians, it's not exactly a place where wheelrunners would be congregating..

Some kind of horizontal deflection — a curve to the side can still be used to slow bikes near an intersection. But here the original design practically required getting off the bike to go through it, while the path around it will hardly slow bikes at all. So both attempts were failures.

Here’s example of a newly constructed protected bike lane which curves as it approaches an intersection to slow bike traffic.