“Why are you teaching 70’s 8051 core ? ” . I was asked this question by students as well as working professionals. But no one asked why are you teaching C and Linux.
Although I knew the answer but decided to do one survey for awareness.

You can observe that closed one is PIC based architecture but I believe that they voted as per end users not semiconductor companies. I deliberately kept ARM and AVR out of this competition. Otherwise ARM would be definitely the winner.
History
I can see some of the devices like 80c35, 80c48, 80c31/32, 80c51/52 etc at Intel Vintage
From Intel website
“In December 1977, Intel engineer John Wharton forgot his wallet and asked a friend to take him to lunch. The friend, manager Lionel Smith, said he had to attend a lunch meeting, but invited Wharton to join him. That free meal would lead one of the best-selling microcontroller series in the world, the MCS-51. “
List of Semiconductor companies who adopted 8051 core
You can easily find this using Keil IDE

I am also putting GIF file but it may take time to load

You may also review devices from various well known semiconductor companies :
Are these devices still being used ?
Check new Avtaar(incarnation) of these devices at Nuvoton . Nuvoton has good sales & support in India.
How 8051 based course is helping in Embedded Developer Kit ?
80C51 is still being taught in many top level engineering colleges and training institutes in India. The main reason is it’s simple architecture and Assembly. One can understand many basic things like Address/Data bus multiplexing, internal/external memory model, ALU, General & Special purpose registers etc.
Here is our offering for Embedded C with 8051 :
Module1 : This covers mandatory C skills required to understand Embedded C . It starts from scratch

Module 2:
8051 core specific architecture, assembly and C practice

Module 3:
Industry importance of 8051 based devices and introduction to next learning path of ARM bare metal.
Here I strongly advise students to purchase at least Nucleo-STM32 board.

Q: You have provided multiple links to learn 8051 , so why anyone will purchase Embedkari 8051 course.
Ans: I have given small example of content quality on web in my earlier article How to Learn C?
I am not selling content here but sharing my experience since 1992, in the form of courses. It comes with cost because I know “No one give value to free stuff” .
How to Purchase ?
You may check Embedded C with 8051 for direct payment link.
About Author
Sanjay Adhikari : A Computer Engineering graduate who got stuck in Microcomputer
•More than 25 years intensive experience in designing, developing and supporting various embedded designs in Telecom access networks.
*Implementation of embedded technical support, Hiring & Mentoring of embedded engineers.
•Over 10 yrs experience in handling an embedded-system support team as a Team Lead.
•Expertise in Design and Development of Linux Device drivers and Bare Board Device Drivers for various telecom/networking devices. Technical knowledge of interfaces like DDR, PCIe, SerDes, Ethernet, I2C etc
•Hands-on experience in Board Support Packages (BSPs) for PowerPC, ARM SoCs from Freescale.
•Enabled Trans-national(Cisco, Alcatel-Lucent, Juniper etc ) and Indian (TATA, C-DOT, DRDO) clients to implement Freescale Networking SOCs.
•Supported all Major Networking (Cisco, Juniper, Aricent) clients in hardware and software troubleshooting on custom hardware. Help customers in porting Freescale device drivers to customer specific Operating Systems such as IOS , JUNOS, FreeBSD etc.
•Provided training to design engineers in using Freescale’s CodeWarrior and Linux BSP.
•Conducted training and demonstration of Freescale technologies in various public events such as Freescale Technology Forum, Design with Freescale etc.
Our ARM bare-metal course has 150+ lessons to cover everything required for Embedded software/firmware .