To turn off Reorder notifications, go to the Alexa app -> then tap the 'More' menu button in the lower right corner -> now go to 'Settings' -> then go to 'Notifications' -> then 'Amazon Shopping' -> then Disable 'Recommendations & Deals'
Turning off Recommendations & Deals should stop Alexa from disable Alexa reorder recommendations of products you've previously purchased on amazon.
UPDATE 5-12-2021: You may also need to change your Alexa settings on Amazon.com here: Alexa Shopping Notifications
NOTE: I had disabled the 'Recommendations & Deals' on the app, but when I went to the amazon link above (Alexa shopping notifcations page) on amazon.com, the Recommendations & Deals' was still Enabled, so I turned it off on the website. Hopefully, that will also turn off Alexa Low Supply Notifications.
I have disabled everything on that page except for Delivery Notifications (Out for delivery / Delivered).