There's a whole bunch of software out there that makes certain
assumptions about allowable ranges. That is, they've been compiled with
a header that defines ..
Of course correct. It really depends on the vendor / software / versions in an environment. A lot of vendors removed that years ago, because frankly a lot of large networks have been using 240/4 as pseudo RFC1918 for years. Others have worked with smaller vendors and open source projects to do the same.
It's consistently a topic in the debates about 240/4 reclassification.