Wow! That looks like a very good start ✨

Few mandatory nitpicks:

• In order to ease the task for the consumers, could you please add a message to the [Obsolete] attributes pointing at the correct overload signatures? You can peek at 0d00a5f for some inspiration 😉
• Please remove all the code beautification changes. They make the diff quite harder to read.
• Could you update the code of the deprecated methods so that they redirect to the newly added ones?
• I may be wrong, but I think you might have forgotten to also consider at least Branch.cs and IRepository.cs (Finding all occurrences of CheckoutModifiers should help you cover all your bases)
• In order to keep the tests up-to-date, could you please also make sure they all leverage the non deprecated versions of Checkout() methods?

Hey @Defavlt! How are things going?

If there anything we can do to help you with this PR, please let us know.

Closing this due to lack of activity.