Is the 1.1x-beta version (backward) compatible with the 1.0.5 release?

Version 1.1 is not BC with 1.0.5, but there will be an upgrade path.
If you start building a WIP, better start with 1.1-beta

thanks @valthebald for the clarification.
Well, actually my WIP was already provided with version 1.0.5, and now it looks that I should re-start over, with 1.1-beta.

Perhaps, in accordance with Drupal's semantic versioning model ( https://www.drupal.org/docs/distributions/degov/semantic-versioning-model → ) and given the discontinuation of BC, a new 2.0.x release would have been a better name for this branch... (?).