The framework needs to parse the HTTP request. That means holding the parameters in a variable somewhere just to arrange them in a datastructure for processing.

But let's ignore that and say we have some kind of system that stream parses the request right out of the buffer (which itself still needs to be held in memory for a bit, but let's ignore that), and when it matches a preconfigured password parameter, passes it directly to the hashing system and nowhere else. I don't think any framework in existence actually does this, but let's run with it.

We'll still need to pass that value by whatever the language uses for function passing. It will be in a variable at some point. Since we rarely write in C these days unless we have to, the variable doesn't go away in the system until the garbage collection runs. Most systems don't use ref counting (and I think it's a mistake to disregard the simplicity of ref counting so universally, but that's another discussion), so that could happen whenever the thread gets around to it.

But even if it runs in a timely fashion, the memory page now has to be released to the OS. Except most runtimes don't. First, the variable in question almost certainly was not the only thing on that page. Second, runtimes rarely, if ever, release pages back to the OS. They figure if you're using that much memory once, you'll probably do it again. Why waste time releasing a page just to make you spend more time getting it again?

And we're still not done. Let's say we do release the page. The OS doesn't zero it out. That old variable is still there, and it could be handed over to a completely different process. Due to Copy on Write, it won't be cleared until that other process tries to write it. In other words, it could still be read by some random process on the system.

And we haven't even mentioned what happens if we start swapping. IIRC, some Linux kernel versions in the 2.4 series decided to swap out to disk ahead of time, always having a copy of memory on disk. Even if you're not running such an ancient version, you have to consider that the kernel could do as it pleases. Yeah, now that var potentially has a long lifespan.

To do what you want, we would need to coordinate clearing the var from the code down through the framework, runtime, and kernel. All to protect against a hypothetical memory attack. Which are actually quite difficult to pull off in practice. It'd be easier to attack the client's machine in some way.

And on top of it, you're running around with an undeserved sense of superiority while it's clear you haven't actually thought this through.