Regarding Remove Startup Delay and Restore Startup Delay at Preferences -> Launching Opus -> Startup, I've found information elsewhere that this is controlled by the value of Startupdelayinmsec at HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Serialize in the registry.
Just fyi, as far as I can tell the cited Preferences buttons don't create or alter this value. It's possible they control the delay in some other way for all I know.
That is exactly what they do modify. I checked and they still modify it in 12.6 and the current Windows 10 build.
Looks like perhaps this has to be done from an Administrator account.
You should not need admin rights to change things under HKEY_CURRENT_USER. I've only tried with an admin account, but UAC is active, and the registry change does not use elevation, so it work the same for a non-admin account.
Are you pushing F5 in RegEdit to refresh the view after clicking the button in Opus? RegEdit does not reflect changes automatically.
Also make sure that RegEdit is viewing HKEY_CURRENT_USER for the user you're running Opus as. If you are logged in as a standard-user account and using over-the-shoulder elevation to launch RegEdit under a separate admin account, then HKEY_CURRENT_USER in RegEdit will reflect the admin account's registry data, not your standard-user account's.
Are you seeing a problem with the button/setting not working, or is this purely academic/curiosity?
I think you are right on all counts.
Yes, it was mostly academic curiosity and yes, I was pushing F5, but I think I was probably doing what you refer to as "over-the-shoulder elevation to launch RegEdit under a separate admin account". I'm not sure how/why I got into that habit, but figuring that out is a Windows issue, not an Opus issue.