[joshka]

They're in the Unicode box drawing block. https://en.wikipedia.org/wiki/Box-drawing_character

[stanac]

I am aware of those, I have used them, but this font has rounded corners on boxes, that's why I believe it is a custom font.

[8organicbits]

The code references FiraCode:

https://github.com/EnhancedJax/Bagels/blob/7b4bfd051f6beff8f...

But the actual character is: ╯

[Thorrez]

256F ╯ Box Drawings Light Arc Up And Left

https://www.unicode.org/charts/nameslist/n_2500.html