It seems unlikely that Sofia will be included in the Level Drain system given Sofia's existing drain scene. In one of her defeat scenes, Sofia will drain all of your stats and levels, stopping just before she drains your soul, just to state that they do not really do anything for her before she eventually returns everything she absorbed. It is quite likely that Sofia is completely maxed out given she has devoured millions of souls, and likely the stats and levels of a number of her victims as well. She is clearly much more physically powerful than her appearance and currently revealed stats may suggest given she is able to crack the floor of the Forest Dungeon with a single stomp during her Collar scenes. While Sofia does state she is not quite as powerful as she was when she was devouring souls, rather than having directly lost power, it could be a situation similar to Arcueid from Tsukihime and having to use a portion of her power to restrain her soul-thirst so that she just does not go on a rampage and start consuming everyone before her given she stopped soul-eating cold turkey.
If anything, Level Drain will probably be specific to a future NPC, and perhaps a few other characters. That way people who enjoy the content can freely indulge in it while those who do not will not have to worry about their efforts leveling up their character being undone. Just like those who enjoy grinding to a degree will be able to use the Rebirth system to continually grind and power up their character somehow while resetting their level to 1 each time they hit the eventual level cap of 100.
While this may be disappointing, I do hope that you find this enlightening,
Gojii.