Hello, I am really happy that my little finding interested you.
This rule is very nice, but there may be too many survival conditions. As a result, when running a random soup a huge "immortal coral" develops. And since these "immortal corals" seem to be emergent, the entire universe is doomed to be filled with voids and "immortal corals".
It is not exactly true. This rule not only preserves the number of alive cells, but it is also time-reversible.
The latter allows us to mathematically prove many facts about the rule, much more than in the non-reversible automata.
Particularly, because of time-reversibility, the immortal coral is not only indestructible, but also "inconstructible".
No interaction with any other pattern: spaceship or combination of them, could ever add new cells to the coral. It's more undead than immortal, not dying and not living. This also means that if a lightest spaceship collides with a coral, then after some time another lightest spaceship will emerge, sometimes after a long time. For me, this process strikingly resembles interaction of an atom and photon: spaceship (photon) hits coral (atom), "excites" it (fragments of spaceship orbiting chaotically around coral, like excited electrons, and finally new spacehip spontaneously emerges. Of course, it's nothing more than curious resemblance.
For example, here is a collision of an orthogonal lightest SS with a bar-like coral, producing the same SS after 6784 generations: Simulator link
Another consequence of time-reversibility is that in any reaction with 1 or more spaceships, at least 1 spaceship emerges. So, this universe is only doomed to perpetual motion: in any reaction with spaceships, at least 1 new one will be created. Here is an example: a spaceship on a field, sparsely filled with random cells
. The spaceship chaotically moves inside debris, oscillating between different forms, and sometimes even splitting in 2, but never gets absorbed.