A way to get a notification (through email for example) when a game is about to be delisted?

Unless a publisher notifies it, theres no way of knowing when a game is being delisted. The only way to know is to visit sites with forums about steam or pc gaming (like here).
Anyway, unless you are a collector you shouldnt think to hard about this, games only get removed when they have licensed content (songs, brands, actors, etc) or when a new version is released to replace the current one, so as long as you keep track of those games or even better, buy them at the first chance of a sale, you should be fine.

You can follow this curator https://store.steampowered.com/curator/31857481-Games-at-risk-of-removal/

Additionally to the curator mentioned by Orionid, there is this group: https://steamcommunity.com/groups/RemGC. You can only join if you already own a specific amount of delisted/purchase disabled games, but there is a public forum. The group also has a Discord server.

There is a sg thread for things like this which you can bookmark.

https://www.steamgifts.com/discussion/37s1B/steam-game-removal-thread-last-update-810-forsaken-world-gods-demons-skald-prologue