Safe to Install New Dual Boot?

It should be safe, although for the future, I'd recommend installing the os to a completely separate drive and changing boot device by uefi.

this reads like you don't make backups or don't have a restore strategy. if the system is important enough that you worry about updates breaking it, you should make backups from time to time and have a bootable restore system on a removable medium.

Dumb title. Yes, it's safe. Windows has nerfed boot records for any other OS since the beginning of dual-booting. Just replace the boot record.

Also, if you want to be hardcore about it, and since everything is UEFI now, just use your BIOS boot manager to control booting. Shouldn't be a problem.

it should be safe. From what I know the update only broke it as it was updating so installing a dual-boot after should be fine. You might want to test first so just install something small like mint for a test and see if it works.

Today on "Was this caused by stupidity or malice"...

Microsoft said earlier this month it would apply “a Secure Boot Advanced Targeting (SBAT) update to block vulnerable Linux boot loaders that could have an impact on Windows security,”

(emphasis mine)

I would advice to just move to Linux all together. No need for dual boot ;)... It took me way too long to do that.

no.