So it seems like block-progression is simply an outdated directive. The more interesting question is whether it should be replaced with another currently-supported one.
I really don't understand anything about CSS, but it might be related to this thing:
Running src/gtk-2.0/render-assets.sh or src/xfwm4/render-assets.sh produces warnings "Ignoring unrecognized CSS property: block-progression"
This seems to have been present in arc-theme already (I'm not referring to the particular bug discussed there, but to one of the messages only): https://bugzilla.redhat.com/show_bug.cgi?id=1982160
It might be related to this:
https://gitlab.com/inkscape/inbox/-/issues/7599
So it seems like block-progression is simply an outdated directive. The more interesting question is whether it should be replaced with another currently-supported one.
I really don't understand anything about CSS, but it might be related to this thing:
https://webplatform.github.io/docs/css/properties/block-progression/
Maybe the block-progression: tb in the svg files should be replaced by writing-mode: horizontal-tb?
https://webplatform.github.io/docs/css/properties/writing-mode/
This seems to be the default value, so most likely ignoring block-progression does not do any damage.