... I have seen no evidence, and can conceive of no reason, why a reversionary downgrade (such as discussed in your cited article but not relevant to the OP) in a Mac could possibly affect the manufacturer's guarantee or future service, when the downgraded OS had previously been used.
That said, trying to boot a 16" MBP from an external disk might be an option. The difficulty is that if the OS on that disk was created for an older machine, it is quite likely to fail.
Jeremy,
On 16" MBP, the only installed OS is Catalina, AFAIK. No other
previous OS has, or will be, installed, again AFAIK.
Apple, I'm told do not, and will not, support previous boot OS's on the new 16" MBP. If that is indeed the case, I'd rather not find myself with a 'blown' motherboard (as I did twice on a 2012 MacMini) with an uncertified/unsupported OS. Not that any of this will affect your ability to run a previous OS in a Parallel's VM. It won't.
I don't follow your comment on creating an external HD/SSD. It's a relatively simple matter to format,
partition and create a bootable disk (
https://support.apple.com/en-gb/HT201372). Copy the installer for the desired Mac OS onto the SSD and Bob's your Uncle ...
Manoli
*Edit:
As an aside, perusing the Apple support site, pointed me to an interesting link
https://www.stclairsoft.com/Go64/free downloadable utility to check which apps are 64-bit app compatible.
More detailed than the built-in SysReport under 'About This Mac'.