I worked multiple years in writing drivers or low level libraries for companies building network cards (mainly in finance). I also do a lot of networking/electronics at home, including building simple network cards. To answer your question "'m not sure how widely used they are anymore": I would say it's almost impossible nowadays to find an ethernet-only chip that does not have any kind of tcp offload ("toe" for short). 90% of the times the chip also handles ARP & ICMP for you. Most recent drivers are able to offload most of the work to the ship, and fallback on software for complementary features (most on-chip TOEs are still barebone) or when no TOE is present.

All Intel NICs do TCP offload, they are quite widely used.

Do you have a source for more information? I know Intel NICs are mostly regarded pretty highly.

I don't know much about it, but it's briefly described here: https://www.intel.com/content/www/us/en/support/network-and-... (ctrl-f offload)

https://wiki.linuxfoundation.org/networking/toe suggests it's done mostly in closed-source firmware.