You can setup MacroDroid app to detect battery saver and notify you about it in the way you want it to be.
Add a new Macro with these conditions:
Trigger: Battery/Power > Battery saver state > Enabled
Actions: as you wish. You can setup a notification with a sound, vibrate, or try other actions.
Save the Macro and try the battery saver manually. MacroDroid should be working for you in that case.
Note: Tasker app can also do the same thing but it doesn't have battery saver state context. Instead, you can define battery range for your context.
Solution tested in OnePlus 6 running Android Pie.