I’m running macOS Mojave and Ardour 8.12.
For a person with this setting always disabled…
-is it normal to see this “Scanning for plugins” window every single startup?:
Is this actual plugin-scanning? I mean, it says at the top it is.
…
Hmm…
Wait.
Actually, you know what?
When I disable this setting, too, only THEN does it not show that “Scanning for plugins” window at startup, which also saves me about 12+sec loading time:
CONCLUSION:
Both “Scan for [new] Plugins on Application Start” and “Always Display Plugin Scan Progress” need to be disabled to NOT scan for plugins at startup (-at least on macOS here). (?)
~Just FYI. 
-J