OK, so the work-around is to say #+asdf3 with UIOP:RUN-PROGRAM and #-asdf3 with ASDF:RUN-SHELL-COMMAND. This isn't ideal, since I think there may be versions that don't have ASDF3 in *FEATURES* where ASDF:RUN-SHELL-COMMAND isn't backward compatible. But it's at least a partial work-around.
OK, so the work-around is to say #+asdf3 with UIOP:RUN-PROGRAM and #-asdf3 with ASDF:RUN- SHELL-COMMAND. This isn't ideal, since I think there may be versions that don't have ASDF3 in *FEATURES* where ASDF:RUN- SHELL-COMMAND isn't backward compatible. But it's at least a partial work-around.