When you read it as 'a massive pile of legacy Windows applications', which is reasonable, it's not a stretch to assume that many applications written in the last 20 years assume that the OS is on a harddisk, and on C: for that matter.
Prior to Windows 95 the preponderance of Windows programs were 16bit. Seeing as 16bit programs are not compatible with recent versions of Windows one can be reasonably assured that they are not in that vast pool of legacy applications.
Hand in hand with 32bit Windows came the registry, which is where most applications stored configuration information until relatively recent times.
To the best of my recollection, I have not seen an application or utility hard coded to C: since Windows 95. But I would be interested to learn of the actual examples which support your assumptions.
Most code was written for internal systems. So even if you don't encounter them there are plenty of hard coded paths including drive letter in plenty of systems.
Hard coded paths are possible with any operating system, including OSX which was mentioned in the post to which I responded. And when dealing with it, assigning C: to the bootable drive is a feature of a particular implementation, not a legacy of the Operating System. So far as I am aware, no part of recent Windows versions are required to be installed on C:
Many games and apps may run on other than the C: drive, but odd things are broken - configuration changes don't stick, temp files fail to be written etc.
Seeing as Windows has been bootable from removable drives and over networks for a long time, I sincerely doubt it.