In the reddit sub you have linked it is described for a solution to enlist into the steam-beta after downgrading the package. Then you have to wait until the steam-patch is done and upgrade the freetype2-package back again afterwards.
I have tried it this way and steam-runtime (beta) works normally for now. I will try to retreat from beta again as soon as valve update the steam-client next time. Maybe then the beta-version won’t be necessary anymore.