Checking JustWatch or Flixable regularly is honestly the most practical method you have, even if it's not perfect. These sites show you when content disappears from the catalog, so at least you find out right away instead of discovering it when you open the app. It's not predicting the future, but at least you won't be caught off guard halfway through.
That said, the previous answers about licensing are correct but they're leaving out part of the story. Netflix doesn't remove series just because it doesn't make economic sense to renew them - they often decide to pull content to make room for new deals, even if a series still had an audience. It's a strategic choice on their part, not always driven by the numbers. What I'd suggest is never start a series without first getting a sense of how "safe" it is - if it ended years ago and has millions of views, it'll probably stick around. If it's a lesser-known series or not heavily promoted, the risk is higher.
The only real solution would be to download the episodes while they're still available, if the platform allows it, or consider alternative services where you might find that series as an exclusive or in a more stable way. It's not ideal, I know, but that's the game all these platforms are playing now.