I understand your concern. The banner was made removable for this reason, but you are correct that if the app is terminated by the system, the banner will re-appear the next time you launch it.
I should clarify that this was a deliberate choice, given that Master Password has essentially gone out of maintenance and it's important that people are continually aware that though they are fully able to keep using it, they do so at their own risk and without official support channels available to back their continued usage. If dismissing the banner were permanent, we'd be confronted with the possibility that people would lose track of this fact and continue to use the app without giving this risky situation much thought, until some issue occurred with which they need help.
Please feel free to let me know having taken this into account, what you feel regarding the current solution and the balance it is aiming to strike between being informative, helpful, and yet allowing people to make their own choices or migrate at their own pace.