Are there any firmware updates, or other changes, done by the 10.14 installer that could cause a 10.10 system to not work?
To quell all the "why would you want to do that?" questions, here's the scenario:
At my work I have a Late 2013 Retina MacBook Pro (model MacBookPro11,3) that, until recently, I kept on 10.10 on in order to use a suite of tools for work that interface with some older hardware. It's the only Mac we have.
We just managed to buy a newer version of the hardware. It's tools are modern and can run on 10.14 without issue. However that older hardware will be moving to another room to be used elsewhere so we still need to interface with it to configure it. The older tools don't support the new hardware and can't run on 10.11 or above, the newer tools don't support the older hardware and require "a modern version of MacOS" (the developer recommends 10.14, doesn't say anything about previous versions).
A VM is not an option as there isn't the ability to do FireWire passthrough to a VM with the ThunderBolt to FireWire adapter.
My plan is to dual-boot the machine with a 10.14 install and a 10.10 install. To configure the new hardware, boot to 10.14. To configure the old hardware, boot to 10.10.
My worry is that running the 10.14 installer will upgrade firmware on the machine to a version that is incompatible with 10.10, and thus the dual-boot would not work.
Best Answer
I haven't heard about such incompatible firmware updates.
I have dual-booted 10.14 (Mojave) and 10.11 (El Capitan) on the 2013 Retina MacBook Pro, which is almost the same configuration - and that worked without problems as far as I recall.