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Not for special VDEVs, which is the explicit purpose here.

You can mirror a slog... l2arc doesn't need a mirror because the data is already on disk. I believe a split metadata can also be mirrored.

That's correct. I ran a special metadata VDEV 3-way mirror using this NVMe PLX card for a while https://imgur.com/a/xiwzkA6

You can dump any vdev you want in a special. I have 2x 3 way mirrors in both of my nas boxes right now