Fuck it, I decided to go back to his old videos to see when the pivot point is.
This clip is dated May 11th, 2019, "GameDev Builds a Chair"
(Full video is 40 minutes),
Archive on PreserveTube
EDIT: Forgot this clip, also uploaded
May 16th, 2019
Now, only a few days later (EDIT: Same day as the previous clip), you can hear some pretty clear variations in the voice,
May 16th, 2019
And, again, three days later, the voice sounds deep.
May 19th, 2019
I'm not saying his voice in the last clip is perfectly 1:1 to today, nor that these clips are timestamped correctly, but as far as what's on his channel, this is the shortest period of time with the most gap in pitch. There's a huge gap in content afterwards, and the next video with him in it (the omori playthrough) has the deep voice.
Interestingly enough, according to the overlay, the last clip is before the third clip. Make of that what you will.
Another thing, which, while misc, is interesting, is that the one and only fight in heartbound
was complete in September 2017.