I'd recommend going to v13, as it's LTS (longer support, see:
https://umbraco.com/products/knowledge-center/long-term-support-and-end-of-life/) and still contains Nested Content... And if you're still on v8 now (2 LTS versions behind, v10 and v13), it indicates to me that this project can't be easily upgraded (either due to technical, operational or budget reasons). Sticking to the LTS release would then make most sense in that regard as well.
And regarding migrating Nested Content to Block List: this is already handled by Umbraco Deploy, uSync Migrations and the AutoBlockList package, so I'm not sure whether it will be something the CMS will get built-in migration support for...