And shells are indeed REPLs (in the sense that they run a loop in which they read, evaluate, and print things).

Though to be fair, REPLs for languages like MATLAB or Julia are likely far more useful from a scientific standpoint than a typical shell (which is more useful for system administration).

Compared with what I know from Xerox PARC documentation and what I self experienced in Oberon and Visual Works Smalltalk, shells are like REPLs yes, but not quite.

It is a very different experience when the shell (e.g. Transcript in Smalltalk) is capable of interacting with the whole OS.

For example in Cedar, which Oberon copied, any public function/procedure exported by dynamic libraries can be used in the shell via modulename.function.

Depending on their signature they act on REPL text input, the active selected graphical element or some other OS element.

While this can kind of be emulated on an UNIX shell, it breaks down that there isn't an OS wide standard API to bring everything together.