The shift key is not preventing opening of document(s) when launching an app.

bill.boland wrote:

open -a /Applications/Safari.app --args -ApplePersistenceIgnoreState YES

Holding down the shift key when launching the app should do the same...but it is no longer.

You can verify your software is up to date

You can reinstall the macOs on top of your existing macOS to sort anomalies

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

if no insight or resolve:

Call Customer Support  (800) MY–APPLE (800–692–7753)

or on line  https://getsupport.apple.com/

or call  AppleCare Support at 1-800-APLCARE (800-275-2273)

Outside the USA—Contact Apple for support and service by phone

See a list of Apple phone numbers around the world.

https://support.apple.com/en-us/HT201232

In many applications, Command-Option-W closes all windows, so that will give you a clean state even if started with several windows open.

bill.boland wrote:

In the past, I have used the shift key when launching an app to begin with a “clean slate” (no prior open documents, files, etc.). This has included Apple apps such as Safari, TextEdit, Mail, Xcode, etc. This is documented (toward the bottom) at: https://support.apple.com/en-us/HT204005

But recently, this no longer seems to be working on my MacBook Pro (2018; Big Sur 11.6). It does seem to work OK on another/older MacBook Pro on Big Sur 11.6

I have tried booting in safe mode, verifying that the shift keys are working properly and trying with both the left and right keys, checking the Keyboard utility in the System preferences to see if somehow the shift keys were reassigned. So far, nothing has resolved this. My last resort will be to reinstall the OS but I was hoping someone in the community has had this and found an easier way to solve this. Maybe there is some utility, preference, or command-line setting that went awry?

Many thanks in advance.

Why the on-going need to "preventing opening of document(s) when launching an app ? "

This is not a normal work flow to begin with....the link you provide above has a more routine way of closing windows:

Prevent apps and windows from reopening

https://support.apple.com/en-us/HT204005

Set all app windows to close when quitting apps · On your Mac, choose Apple menu > System Preferences, then click General. Open General preferences...

Quit apps on Mac - Apple Support

Are you having associated issues, maybe you can say more?

It is interesting that this does work in one mac and not the other.

Actually, I just tried this and on my mac (16" MBP, Big Sur 11.6) it is also not working.

I never use this, that is why I had not noticed it, so I can't say if this changed recently or not.

Anyway, "what could be different between your two macs?" seems to be the question to ask.