Removal rotates the read key: a new key is sealed to each remaining member, later envelopes use it, the removed member keeps what was delivered, and old keys are carried under new ones.

It is what removal guarantees, and what it cannot.

Every member holds the same read key. It opens every envelope in the space.

Now remove a member. Crossing them off the list is not enough. They still hold the key.

So removal rotates the key. A new read key is made. A copy is sealed for each remaining member, with their public key.

Every envelope written from now on uses the new key. The removed member cannot open any of them.

What they already received stays readable to them. Encryption is forward-looking. It cannot take back what was delivered.

And the old key is carried forward, sealed under the new one. A newcomer gets the newest key, and through it, every key before. They can read the whole history.

Check yourself. After removal, which envelopes can the removed member still read?