TL;DR: This is the first half of the heap.  We don't waste it.

On F1, the heap is split into 2 parts during early init:
* The "pre-rtos" sized block seen here is what's available during board init and module init so that drivers and tasks can allocate their stacks.
* The "init stack" (a couple lines above) follows this and is used as the stack for the init code.

After init has completed, we flip the SP to the IRQ stack (totally separate from the heap), "free" the init stack by basically just extending the FreeRTOS heap all the way to the end of RAM, and start the OS and all of the tasks.

All of that was done ages ago to save some RAM on F1 by reusing the init stack as heap.  See also my previous comment about us now leaking the init task's stack on F3/F4.