There is a proposal to add some of the old missing box drawing characters to Unicode, in a section called "Graphics for Legacy Computing":

http://www.unicode.org/L2/L2017/17435r-terminals-prop.pdf

They have been implemented recently in the Pragmata Pro font I use, which seems to just keep adding glyphs

https://www.fsd.it/shop/fonts/pragmatapro/

Unicode still contains a large number of box drawing characters. Some are broken into three, or even four parts.

https://en.wikipedia.org/wiki/Box-drawing_character