In the past couple of days, news sites such as ZDNet and iMore have reported that macOS can display a notification advertising Safari when you first launch Microsoft Edge. It turns out that this "feature" actually appeared first in Mac OS X 10.10 Yosemite, as described in an old blog post by Daniel Aleksandersen, an engineer for the web browser Opera. The Safari advertisement can occur with any alternative web browser, such as Opera, not just with Microsoft Edge. Ironically, it can occur even with Apple's own Safari Technology Preview! I've discovered reliable steps to reproduce the advertisement, using the information from Aleksandersen's blog post.
Set your default web browser to something other than Safari.
See my blog post from a few days ago describing a bug in macOS Big Sur that can prevent you from changing your default web browser, and my new free app Default web browser that works around the bug.
Launch the Terminal app in the Utilities subfolder of the Applications folder, and enter the following command.
defaults delete com.apple.coreservices.uiagent
(You can use defaults read com.apple.coreservices.uiagent at any time to see the current defaults.)
Logout of your user account and login again.
Launch a web browser other than Safari, and then quit that browser.
defaults write com.apple.coreservices.uiagent CSUIRecommendSafariNextNotificationDate -date "2021-01-01 00:00:00 +0000"
Logout of your user account and login again.
Launch a web browser other than Safari.
And that's it! Enjoy your beautiful macOS advertising. But if you want to prevent the ad from ever popping up again, enter the following commands in Terminal, and then logout.