Nobody claimed that "sum types are be all end all". I originally responded to "how would you handle cases where a value is unset without NULL" with "sum types" which are trivially presentable with bit masks if memory usage is of big concern (and nowadays in 99.9999% of the cases it genuinely is not).

And, tagged unions are a thing and were a thing for a long time.

Of course it's too late to change all this today; it would have been too late even 20 years ago. But outside of f.ex. Linux kernel and some other super hardcore C libraries, a lot can be done for the world to migrate to safer constructs and away from sentinel values. And that's what languages like Rust do.

Super memory constrained environments have not been the mainstream programming work for decades and now remain limited to embedded / IoT. Not sure what the reservation against sum types is these days.