It seems obvious enough that questions about an Osborne 1 computer would be on-topic here, but what about new machines built using old 8-bit microprocessors? There are still computer kits available based on the Z80, 6502, CDP1802, etc. Occasionally, these can mix with newer parts to provide capabilities such as TCP/IP network access or interfacing to Arduino shields.
Many of these old microprocessors are still manufactured, so it's possible to create an entirely new computer with a core architecture envisioned in the 1970s.
I'd posit that such machines still qualify for retrocomputing discussion, as it is the design rather than the fabrication date of the silicon that truly dates the CPU, but it may be worth some discussion.