The "Microsoft BASIC Compiler, Version 5.3 July, 1981" manual https://archive.org/stream/BASIC_Compiler_v5.3_1981_Microsof... says:
"The compiler operates under the CP/M operating system"
"6.3 COMPILER SWITCHES ... must begin with a slash (/)"
"MACRO-80 ASSEMBLER ... 2.2.2 Switches ... must be preceded by a slash"
"4.2.1 LINK-80 Switches ... must be preceded by a slash (/) . (With the TEKDOS operating system, switches are preceded by hyphens . See Appendix A.)"
"5.2 LIB-80 SWITCHES ... are always preceded by a slash"
(The "TRS-80 Manual: BASIC Compiler" https://archive.org/stream/BASIC_Compiler_19xx_Microsoft/BAS... says:
"3.2.3 Command Line Switches ... Switches are always preceded by a dash ... -Z-4-T"
"Filename extensions must be separated from the filename by a slash mark (/) ... /BAS Basic source file"
and does not mention CP/M.)
"Microsoft FORTRAN-80 Ver3.4 Users Manual" https://archive.org/details/bitsavers_microsoftc0Ver3.4Users... also says switches "must be preceded by a slash" for CP/M, but for TEKDOS (Section 4) "the switches are delimited by commas or spaces instead of slashes"!
"Microsoft FORTRAN-80 Users Manual 1977" https://archive.org/details/bitsavers_microsoftc0UsersManual... also already says "Each switch should be preceeded by a slash (/)". "Section 5 Operating Systems" says command lines are supported on "CPM" and "DTC Microfile", but not on "ALTAIR DOS".
"Microsoft COBOL-80 1978" https://archive.org/details/bitsavers_microsoftc978_6774127/... says:
"Each switch must be preceded by a slash (/)" (1.1.2 MACRO-80 Assembler, 2.1.2 LINK-80 Linking Loader)
"switches are always preceded by a slash" (3.2 LIB-80 Library Manager)
"Section 4 Operating Systems" Command lines are supported on "CPM", "DTC Microfile" and "ISIS-II", but not "Altair DOS".
What I see when reading this set of links is that Microsoft really likes slash the the option separator. And while they ran on CP/M neither the CP/M commands[1] or the ISIS commands[2] (both operating systems that Gary Kildall supplied from his company Digital Research.) Used the slash character for options. And that tells me that it wasn't Gary's idea to have slash as an option introducer.
And as mentioned previously, it was DEC's idea to use slashes for options in DCL, which may have been inspired by IBM's Job Control Language (JCL).
The "Microsoft BASIC Compiler, Version 5.3 July, 1981" manual https://archive.org/stream/BASIC_Compiler_v5.3_1981_Microsof... says: "The compiler operates under the CP/M operating system" "6.3 COMPILER SWITCHES ... must begin with a slash (/)" "MACRO-80 ASSEMBLER ... 2.2.2 Switches ... must be preceded by a slash" "4.2.1 LINK-80 Switches ... must be preceded by a slash (/) . (With the TEKDOS operating system, switches are preceded by hyphens . See Appendix A.)" "5.2 LIB-80 SWITCHES ... are always preceded by a slash"
(The "TRS-80 Manual: BASIC Compiler" https://archive.org/stream/BASIC_Compiler_19xx_Microsoft/BAS... says: "3.2.3 Command Line Switches ... Switches are always preceded by a dash ... -Z-4-T" "Filename extensions must be separated from the filename by a slash mark (/) ... /BAS Basic source file" and does not mention CP/M.)
"Microsoft FORTRAN-80 Ver3.4 Users Manual" https://archive.org/details/bitsavers_microsoftc0Ver3.4Users... also says switches "must be preceded by a slash" for CP/M, but for TEKDOS (Section 4) "the switches are delimited by commas or spaces instead of slashes"!
"Microsoft FORTRAN-80 Users Manual 1977" https://archive.org/details/bitsavers_microsoftc0UsersManual... also already says "Each switch should be preceeded by a slash (/)". "Section 5 Operating Systems" says command lines are supported on "CPM" and "DTC Microfile", but not on "ALTAIR DOS".
"Microsoft COBOL-80 1978" https://archive.org/details/bitsavers_microsoftc978_6774127/... says: "Each switch must be preceded by a slash (/)" (1.1.2 MACRO-80 Assembler, 2.1.2 LINK-80 Linking Loader) "switches are always preceded by a slash" (3.2 LIB-80 Library Manager) "Section 4 Operating Systems" Command lines are supported on "CPM", "DTC Microfile" and "ISIS-II", but not "Altair DOS".