Can Stupefy or any reaction cards be cast after damage has been assigned/claimed in a showdown?

Ruling: No, there is no window after combat damage is dealt to play reaction cards.

Nuances:

• Exception: If a unit dies due to combat damage and triggers a deathknell ability, you can respond to that deathknell trigger (the trigger will likely be finalized to the chain in a cleanup after combat cleanup)
• The exact timing of deathknell triggers may vary depending on head judge interpretation at different events