Regarding AbstractModel and save/load methods. Magento doesn't recommend to use them, because these methods violate Single Responsibility principle (S from SOLID). Entity/Domain Model which destiny is to be responsible for some business operation shouldn't be responsible for self-retrieval and saving from/into the data storage.
These methods are marked @deprecated in the codebase
We expect that Entity should be Extracted with the help of Repository API. Which implementation uses Entity Manager or successor of ResourceModel under the hood.
In general, Magento recommends to use interfaces/classes marked with PHP Doc @api, which means that we consider these entities as a part of module's API contract.
For now the coverage of entities in a codebase with @api annotation is not full. So, we can't say that it's enough to use just @api to proceed with all possible business operations (usage/extension/customization) for successful integration with Magento modules.
That's why Magento considers all the code as public APIs now, and apply Backward Compatibility policy for the whole codebase with each release, to protect 3rd party extensions from being broken after update to new Magento version.
That's why we are going to inspect how 3rd party extensions use Magento modules (what Magento entities being used which are not marked as @api) and add @api annotation to all those entities, if there is no existing valid API which should be used instead.
As soon as we make this demarcation, we would start to consider code which not marked as @api as Private implementation. Thus we will modify it without applying Backward Compatibility policy. All the @api entities will follow SemVer.