According to this list: https://github.com/backdrop-ops/contrib/issues/34 Inline Entity Form has not been ported. I've done some looking around and can't find this or anything like it.
I am using the node reference module to link nodes to each other. It would be great to be able to create a new 'referenced node' from within another node edit form, instead of needing to exit the node you are creating, create the node you want to reference, then return to the original node and finish it.
Any thoughts on the best way to do this in BackdropCMS today?
Is the Inline Entity Form module a useful module to port - or does BackdropCMS need a custom solution for this?
EDIT (06/22/2022): Inline Entity Form is available for Backdrop CMS - https://backdropcms.org/project/inline_entity_form
You might also try References Dialog. I've used it a lot in Drupal, and I prefer it to Inline Entity Form. The module allows (while you're in a node form) to create or edit other nodes in a modal window. Haven't tested the Backdrop version so far, but according to https://github.com/backdrop-contrib/references_dialog/issues/5 "it works well enough".