Though neither is currently usable in Scarlet/Violet, the Smogon community has been using them for quite a while in its National Dex Ubers Tier. While it is not a 1 to 1 substitute for the normal metagame, the outcome won't be too different in terms of how good they are comparatively.
According to the National Dex Ubers Viability Rankings, Zacian-Crowned is in A+. While not nearly as good as its former self, it still has base 150 attack and 148 speed coupled with the best typing in the game, plus a fantastic offensive movepool that can cover most things. In Gen 9, it will likely be ranked higher without Primal Groudon dominating the tier.
Meanwhile, Zamazenta-Crowned is stuck in D rank, meaning that it is completely unviable. Whatever niche it used to have in Sword/Shield is gone. The top 4 Pokemon in National Dex Ubers are Primal Groudon (kills it) Arceus-Ground (kills it) Koraidon (kills it) Necrozma-Dusk-Mane (walls it), which should tell you how bad it is currently. The absence of Primal Groudon in Gen 9 helps, but not by a whole lot.
Both Hero of Many Battles forms are unviable, Zamazenta more so than Zacian. Their stats just aren't that great for Ubers. Zamazenta is actually in National Dex OU, serving as a powerful wallbreaker there.
In VGC, there isn't as clear a guideline as in singles, but we can reasonably assume that the rough rankings are still Zacian-Crowned > Zamazenta-Crowned > Zacian-HoMB > Zamazanta-HoMB, as they were in Gen 8. The nerf to their abilities should hurt them less since switching is less common in VGC, but it is still too early to tell.