This is a hobby computer based on the 65C02 processor. (I use 6502 and 65C02 interchangeably in this document, but there are some differences)
There are already many computers based on the 6502 processor, old ones such as as the Apple 1, Apple II, Commodore 64 and many more; and many more recent ones, from homebrew affairs on prototype board, to complete replicas of old systems.
The one that most closely resembles the Planck is probably the RC6502, itself based on the RC2014 (which uses a Z80 processor) with adaptations to the expansion bus to make it fit the 6502 processor.
Planck is a new variant on this type of expandable machines with a different set of tradeoffs.
(From: https://planck6502.jfoucher.com/)
I really like the genre of small, hobbyist built computers, especially with a retro vibe. I need to actually learn how to use KiCAD one of these days and start on a similar project.