migration plan from Drupal 7 rules to Drupal 9 Business Rules

The OP seems to have some misconceptions.

An 8.x branch in Drupal does NOT mean that a module is only compatible with Drupal 8. In the case of Rules, the Rules 8.x-3.x branch is compatible with both Drupal 8 and Drupal 9, and has been compatible with Drupal 9 since shortly after Drupal 9 was released. It says that explicitly on the project page.

For the transition to Drupal 9, Drupal modules were expected to be compatible with both D8 and D9 at the same time. Modules were not allowed to make a 9.x branch for Drupal 9 compatibility - that all had to be done on the 8.x branch. Semantic versioning was introduced later. Because semantic versioning REQUIRES a change in major version number, the switch to semantic versioning is not supposed to be done unless a major version change is required - for example, when BC-incompatible API change is made. Because Rules did not change its API between Drupal 8 and Drupal 9, no change to semantic versioning is needed or desired for the Rules module.

Yes, Rules 8.x is "not yet complete", but neither is Business Rules, both 8.x and 2.x. And Business Rules isn't even being actively maintained anymore, but Rules IS. So what's your point?

It's going to be very hard to migrate Rules 7.x to Business Rules 2.x because there is not a 1-to-1 mapping of concepts. They are different modules. Your best bet is to re-create all your rules manually.

@Neeraj420: The Rules module cannot have a stable release until these issues have been solved: https://www.drupal.org/project/issues/search/rules?status%5B%5D=Open&iss...

If you require it for your project then I suggest jumping in yourself, or hiring developers/freelancers to work on these issues