Rufus author here.
As explained here, Rufus does not embed any version of MS-DOS with the application, so it doesn't matter what version of Rufus you use, you won't magically get a specific version but the Windows Millennium DOS version that Windows provides (that is, up to Windows 8.1, since Windows 10 removed it altogether, so there's no MS-DOS creation option in Rufus if running on Windows 10 or later).
Now, your expectation is that Rufus can create an MS-DOS bootable USB if you provide your own version (as an ISO, or something else). This is not the case. Rufus cannot be used to create an MS-DOS bootable USB drive except a Windows Millennium one, if you are running Windows 8.1 or earlier, and, because the demand for such a feature is exceedingly small (at this stage, I have to point out that the amount of people who have been able to claim that they have found a genuine flaw in FreeDOS that forces them to use MS-DOS currently stands at exactly zero, as most of the "issues" that people have with FreeDOS only have to do with a slightly different way to handle AUTOEXEC.BAT
/CONFIG.SYS
or menus, which is easily addressed and does not constitute a DOS incompatibility), I have no plans whatsoever to ever add that functionality as I have 100% confidence that FreeDOS can always be used in lieu of MS-DOS.
The only way I would ever consider adding this feature is if the following conditions are met:
- You can demonstrate that you have found an actual incompatibility between FreeDOS and MS-DOS (rather than a "belief" that there may exist one, which, in years of being subscribed to the FreeDOS mailing list I have yet to see).
- The FreeDOS developers indicate that they are unwilling to address this incompatibility.
Short of these two conditions being met, I will respectfully request that you either use FreeDOS (which, again, I have yet to see fail compared to MS-DOS in terms of compatibility) or another utility than Rufus.