PL/I or Programming Language One is another legacy language that’s become very niche. Designed for scientific, engineering, business, and system programming, this procedural, imperative language is now only used to maintain old systems or convert them to run in new hardware. 

PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. It has been used by academic, commercial and industrial organizations since it was introduced in the 1960s, and is still used.PL/I's main domains are data processing, numerical computation, scientific computing, and system programming.