-keys expire from servers by default. perhaps 6 months or 1 year.
Removing from keyserver is bad idea. After public key was issued,
there are two status for public keys which are "Valid" or "Not-Valid"
(removked).
Alice, Bob and Olive story.
Step 1: Alice distribute her public key to the world for verifying
her signed text by everyone.
Step 2: Alice sign text and distribute signed text to the world.
Step 3: Bob get Alice's public key and verify Alice's signed text.
Bob is certain that this text was written by Alice.
Step 4: Alice remove her public key from all of the world.
Step 5: Olive get Alice's signed text. But she can't get Alice's
public key and can't verify Alice's signed text.
Olive is not certain that this text was written by Alice.
Step 6: Bob says "this is Alice's text", Olive says "I'm not sure
this is Alice's text".
This is a kind of chaos.
--hironobu