In origin, we used the custom* prefix for things that had both a preset and custom colors added by the user. For example, color.customGradient means the users can't use the custom colors in gradients, they are limited to preset gradients. However, color.link means the users can't set link color.

We should remove the custom* prefix those properties that don't need it: spacing.margin, spacing.padding, typography.lineHeight.

cc @mtias @youknowriad @mcsf @jorgefilipecosta @gziolo @aaronrobertshaw @ramonjd

We can implement the migration from v1 to v2 for the properties above, as we did in the plugin when we introduced v1 (see), so in theme.json you'd use the name without the custom prefix.

I wonder how this affects the "internal" names: ideally, the names of the properties in the __experimentalFeatures passed to the client can be also updated as well as the store. I understand this may require 3rd parties that hook into the block settings filter to update their filters, if they modify these values. Is this a reasonable request? I think it is, because a theme.json v2 could introduce shape changes anyway.

We should perhaps leave the renaming for when we stabilize and remove the __experimental.