The term "V42 2021" typically refers to a specific iteration of password recovery tools or firmware exploits that gained traction in the automation community around that time. These tools are often designed to bypass or remove the security locks on specific brands of PLCs and HMIs (such as Siemens, Mitsubishi, Allen-Bradley, or Delta).
The golden keyword is a siren song. In 99% of real-world cases, the cheapest and fastest solution is: plc+hmi+password+unlock+v42+2021