There is also the recently announced Pentax K-3 III Monochrome [1], but it's a DSLR not a rangefinder. At $2200 I don't know if you'd consider it a "poor man's" camera, but it's certainly cheaper than the Leica.

[1] https://petapixel.com/2023/04/12/pentax-unveils-k-3-iii-mono...

Apparently it isn't impossible to place CMOS under laser etcher to destroy Bayer color filters, causing it to become a panchromatic CMOS sensor(?)[0].

Or I think one could argue that Foveon X3 sensors are monochromatic... "More like YCbCr" or something.

1: https://hackaday.com/2021/08/09/using-a-laser-to-blast-away-...