I am aware that IEEE 802.3 specifies a half duplex mode for gigabit Ethernet that includes things like carrier extension to prevent the collision domain from shrinking due to the higher data rate...but has this half duplex mode ever actually been used before? It seems like many NICs do not even support it. And I am also not aware of any gigabit hubs, only 10/100 hubs.

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All the NICs actually support half-duplex since autonegotiation is a requirement for 1000BASE-T; you just can't manually configure it. As far as I know, there are no hubs for 1000BASE-T, which is where half-duplex would be used.

There was a strong movement not to include half-duplex in the 1000BASE-T standard, but it was left in to support hubs. However, half-duplex was left out of the 10GBASE-T standard.

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