I remember there being a point where SI was phased out of public
interfaces to be replaced by EXT. However, I still see some symbols
shown as being in the SI package, for instance in the APROPOS results.
The SI package is not going to be empty. The functions in SI do exist and might be used, but are not intended for public use -- they are not stable in the sense of future-proof. SBCL also has internal packages and their output is shown in APROPOS. Would you vote for hiding those results?