If you already know what apps are causing the toasts, then find them in settings, apps, show all, show system, and then deny permission to overlay.
If you are not using these apps anyway, just disable them.
If you are using them, then you may be stuck with it.
This is but one reason why I avoid 'security updates' like The Plague.
Usually it is only forced updates to useless app I don't even want, and as an extra bonus they get permissions and 'features' that make me want them even less.
You may be able to downgrade these apps to their previous state, but all data for them will be erased.
Also, it is a bit involved and tedious.
I use Uptodown,
https://en.uptodown.com/ (the app is needed) to find the app(s) and then try to install a version earlier than what is on the device.
You cant do that with a system app, but it CAN erase any updates to that app if you try to uninstall.
(Installing another version is not needed, just the deletion of the updates.)