Reading this doc
https://github.com/esp8266/esp8266-wiki/wiki/Memory-Map
there is a region of memory, called DRAM:

User data RAM. Available to applications.

It is 80 KB.

Does the micropython firmware use it?
If not, is it possible to merge this region the the heap?

MicroPython v1.20.0 on 2023-04-26; ESP module with ESP8266

At startup I have about 35 KB of free heap, running only this boot.py

from micropython import alloc_emergency_exception_buf
alloc_emergency_exception_buf(100)		# give traceback info when RAM allocation would fail

import gc
gc.enable()
#gc.threshold((gc.mem_free() + gc.mem_alloc()) // 4)
gc.threshold(gc.mem_free() // 4 + gc.mem_alloc())

import esp
esp.osdebug(None)
esp.sleep_type(esp.SLEEP_NONE)

from machine import freq
freq(160000000)				# CPU at 160 MHz

try :
	from sys import tracebacklimit
	tracebacklimit(10)
except :
	pass

#import uos
# detach REPL UART (GPIO1, 3; disable print, also via USB)
#uos.dupterm(None, 1) # disable REPL on UART(0)

gc.collect()
print("boot.py", gc.mem_free())