Stata 15 help for version_missing
[P] version, missing -- Set version to use modern treatment of missing
version #, missing
version #, missing: command
version #, missing requests the modern treatment of missing values.
missing is allowed only when # is less than 8 (because otherwise, modern
treatment of missing values is implied).
Before version 8, there was only one missing value (.). To keep old
programs working, when version is less than 8, Stata acts as if . = .a =
.b = ... = .z. Thus old lines, such as "...if x!=.", continue to exclude
all missing observations.
Specifying missing will cause old programs to break. The only reason to
specify missing is that you want to update the old program to distinguish
between missing-value codes and you do want to update it to be modern in
other ways. Few, if any, programs need this.