10mbit/s half duplex. 
Microchip’s ENC28J60 is freely available, used in many things, and only really support 10 Mbps half-duplex. It technically supports full-duplex, but without auto-negotiation!
It was just an exaggeration of the standards' situation. There always is someone who sells what is demnded.
IMO there's one major reason for using 10Mbps ethernet chipsets these days: price.
And IMO there's one pretty good reason to use legacy ethernet in various IoT/home automation devices: power consumption (which is lower for slower rates). This one is important in particular for devices which are not powered by mains (directly or indirectly), but are rather battery-powered.
But IMO this problem is not as big as the problem with WiFi ... mainly because UTP-wired ethernet is point-to-point and switches are pretty good at working with different speeds per port (let's forget about broadcast/multicast traffic for a moment) ... while WiFi is actually using shared medium ... so "single collision domain" and half duplex (similar to old coax-based ethernet 10Base-5 and initial UTP-based ethernet 10Base-T with hubs rather than switches).
That is a point. Keeping "b" standard It's like allowing horse carriages on highways. Four wheels, comfortable leaf suspesion, low emmision so why not? 
Yeah. PM 10M emissions