Horizon is not a new theme so much as a new way of building them — blocks nested inside blocks, up
Horizon is a block-based theme system supporting up to eight levels of nesting, not simply another theme in the store.
The practical shift is that layout composition moves from developer to merchant, which changes who is blocked on whom during a build.
Deep nesting is powerful and easy to abuse — a page assembled eight levels deep is hard to maintain and easy to make slow.
Existing Online Store 2.0 themes keep working; this is an opportunity to evaluate, not an emergency migration.
No. Existing Online Store 2.0 themes continue to work and are not deprecated by Horizon's arrival.
The full guide has the formats, the deadlines and the worked examples.