I had heard that Zetalisp had this feature, but apparently it's older than that — this was how packages originally worked. The 1979 Lisp Machine manual says:
The colon character (":") has a special meaning to the Lisp reader. When the reader sees a colon preceded by the name of a package, it will read in the next Lisp object with
packagebound to that package.
I don't know why CL degeneralized the package prefix to only work on symbols. The only reason I've heard is that a misplaced colon could accidentally cause the next form to be read in the wrong package, but that doesn't sound more dangerous than other typos like stray parentheses.
Old Lisp manuals are a fascinating mix of futuristic and primitive. Lisp Machine Lisp also had hierarchical packages: package names were symbols, not strings, and could therefore live in other packages. But there are no earmuffs on
package, nor on any other special variables; apparently they hadn't been invented yet.