[1] It was a simple command interpreter, not designed for scripting, but nonetheless introduced several innovative features to the command-line interface and led to the development of the later Unix shells.
The shell's design was intentionally minimalistic; even the if and goto statements, essential for control of program flow, were implemented as separate commands.
[2] As a result of the simplistic design, by the 1975 release of Version 6 Unix, it was becoming clear that the Thompson shell was inadequate for most serious programming tasks.
At this time, the developers of the Programmer's Workbench UNIX distribution, most notably John Mashey, began modifying the Thompson shell to make it more suitable for programming.
Since virtually all modern Unix and Unix-like systems are descended from V7 and 2BSD, the Thompson shell is generally no longer used.
It is, however, available as open-source as part of several Ancient Unix source distributions, and has been ported to modern Unices as a historical exhibit.