When you get Steam key revoked (had been activated before), do you receive a notification?

Yes, you do.

I had booster trooper removed but I don't know when and I was not notified.

Not every time.

I got my keys revoked directly from the developer and I didnot receive any message, the game just disappeared from my library, silently.

most of the time according to other people, but my key of Awesomenauts was revoked one time and I did not get notified...

AFAIK, never. I randomly lost Terraria from my steam library once, got no notification, just noticed when one day I could play it, the next it was gone. Luckily someone was kind enough to buy me a second copy. EDIT: I still don't know why it was revoked, the guy bought me the copy within the hour, and my support ticket to this day (it happened roughly 1 year ago exact.) still has not been answered.

This is the message I got when I had a game revoked

silently. I really wish SAPI is still up because it tracks if i lost a game or not.

Is t here any site that does this as well?

Yes, you get a big green notification on top.