Here's the response from Google support for those interested...
Thank you for contacting Google Workspace Support. My name is Courtney and I will be assisting you today. I understand that you have an issue where there's now a banner when delegating accounts, and you would like to know how to remove that banner since it impedes on the ability to do stealth delegation. The banner says:
“ <user> now has delegated access to your account. This notice will end in 7 days."
Thank you for those details and for sending the screenshot. I researched internally and I confirmed that this banner was actually recently implemented by our Product Team to ensure that users were aware of when their account was being delegated. This began to be rolled out several months ago and other customers have also inquired about this banner, once it became implemented for them.
There was an internal ticket created at that time, and it was confirmed that it is now the expected behavior for the banner to show when an account is delegated, in an effort to increase transparency and security around delegation usage. But I have reached out in that internal ticket to see if there are any workarounds available and I will update you when I have any response. But previous cases have pointed to the only viable workaround being to use Google Vault to view the users’ emails, but I know that this may not meet all of your use cases.
Other customers were also recommended to create a Feature Request for this ability to disable the banner, but I have not yet seen that any customers have filed a Feature Request. But I will let you know if filing a Feature Request is the best way forward (and I can file that on your behalf) or if there are any other available workarounds.