The new endpoints are in a 'Preview' version -- we're confident enough to release them to the public and encourage developers to build off of them, but we reserve the right to make minor tweaks without as advanced notice as we'd do on the legacy API.
We 100% encourage development on the new APIs, but we'd be more comfortable if developers were open to adjusting to minor tweaks until we launch the full version (although, we don't anticipate making any changes).
We are working hard to release more endpoints to make sure the new version has 100% coverage over the old API functionality (even on the more obscure endpoints), and then we'll communicate a deprecation schedule. Don't worry though, we'll include a long and fair deprecation period.
Cheers!