By chance I discovered via
zpool status -i that the vdevs on my several-day-old pool were uninitialized. ZFS has been functioning just fine.
Should I initialize my vdevs? Why or why not?
When does it and when does it not matter?
man zpool-initialize is not very helpful:
Begins initializing by writing to all unallocated regions on the specified devices, or all eligible devices in the pool if no individual devices are specified. Only leaf data or log devices may be initialized.
Google is no help. ChatGPT suggests that it can help new block allocations go faster and that it can prevent recovery of old private data that may remain on the device from a previous file system. Those explanations both seem plausible, but I don’t have enough confidence in ChatGPT to rely on it being correct.