That's an interesting attempt at defining L2s imo, although it doesn't fully aligns with mine.

To me, a L2 requires all the properties that BM outlines, plus the ability to exit unilaterally. If you can't exit alone then it's not a protocol layer, it's a service built on top of Bitcoin.

Now what is a bit tricky is to all agree on what "unilaterally" means. We could define it as "as unilaterally as on Bitcoin L1" but then nothing is a L2 if we adopt this definition in its strictest meaning.

Protocols that are the closest to being L2s under this definition are protocols where the user is given a window during which they need their exit transaction to be confirmed, in which case they're able to exit even if other participants are malicious. That's Lightning & Statechains today. Ark and Coinpools in theory, but with current opcodes they're not very practical. Probably many others.

What's your definition of L2? What does it take in your opinion for the exit to be unilateral, if it's a criteria at all? Do you have other requirements in mind?