> It was actually mandated for many government projects for a while
I have no clue about this, but I suspect that the government, rather than mandating a software development methodology, simply used to agree beforehand on what was to be delivered and expected to have it delivered at the end of the process. It's a constraint on how you sign contracts with external suppliers, not on how you write code- though I understand that the first influences the second.
In many industries as well as many government branches it still works just like that. They do not mandate a way of "writing the code", but software projects must pass gates and receive approvals at each stage before getting funding. They have to provide comprehensive analysis and architecture documentation that cover everything under the sun before a single line of code is written. And then, once a dev team starts coding, it has to go through the whole loop and if something specified doesn't make sense "on the real world". Or, more often, they just do the proper thing without telling anyone.
I have no clue about this, but I suspect that the government, rather than mandating a software development methodology, simply used to agree beforehand on what was to be delivered and expected to have it delivered at the end of the process. It's a constraint on how you sign contracts with external suppliers, not on how you write code- though I understand that the first influences the second.