Aside from Bootstrap's alerts page I am unable to find any material guidelines for alerts that we can use in the development of a notification bar.
Are there any good guidelines out there based on user behaviour?
Under the Snackbar section, it recommends which component to use based on the amount of interruption or attention that's needed from the user:
I don't know your use case, or your definition of 'Alert', but you have two other options:
Alert dialogs interrupt users with urgent information, details, or actions.
A banner displays an important, succinct message, and provides actions for users to address (or dismiss the banner). It requires a user action to be dismissed.