Postscript: I just managed to boot OpenWrt 23.05.4 on my N750, probably out of sheer luck that my device is not one of the afflicted ones. And I immediately noticed ...
... that there still is no DSA on ath79. This means that for my personal usecase ("switch and/or supplementary access point") there is little to no benefit of running a current OpenWrt over the last "squashfs bug free" 19.07.
That being said, I feel that the bug still should be squashed (pun intended), as a 16/128 device it still has some life left in it.