This is a variable found in Unix interactive tclsh.Detail on this page the purpose for which it is used.
DGP 2003-05-14: tcl_libPath has only one documented use. It holds a list of directories, and the C-level encoding documentation says that if you add another directory to that list, then the new directory will be searched for *.enc files that hold additional encodings for Tcl to recognize.
But... that documentation is false. This means of extending the encoding search path does not work. Never has. See Tcl Bug 463190.
So,... Tcl programmers should just pretend tcl_libPath does not exist. It cannot be used for anything useful. Treat it as an internal detail of Tcl, not for your use.