As far as I know, Purism is still accepting orders for Fi (if you write Purism and say that you want to change your order from Evergreen to Fir). On Nov. 21, 2020, Purism’s @joao.azevedo confirmed that Purism is still planning to release Fir after Evergreen.
NXP still hasn’t started mass production of the i.MX 8M Plus or even published its data sheet, so I suspect that is the reason why Purism hasn’t said anything about Fir’s design and has not confirmed that it will use the Plus processor.
Given the delays from NXP in releasing the Plus and how long it will probably take to get mainline Linux support for the new features in the Plus, I think it likely that Fir will end up using the i.MX 8M Quad, but increase the RAM to 4GB and the eMMC to 64GB or 128GB. I would love to see other changes in the design, like moving the microSD and cellular modem to a USB 3.0 bus and the addition of a USB extension port in the back with pogo pins.