I wish to automate management of my Build/Release definitions, for easy backup/restore operations.
How can I do this in TFS 2018? I'm open to using either the REST API or the SDK, although I prefer the SDK whenever possible.
--EDIT--
This is for TFS, not VSTS
In TFS 2018 and later versions, it supports export/import build/release definitions directly. Build/Release definitions are implemented internally as .json files, so you can see details on changes in the file’s history:
Check the following release note:
https://docs.microsoft.com/en-us/visualstudio/releasenotes/tfs2018-relnotes#buildimport
Or you can use TFS REST API to get build and release definitions:
Adding a screenshot of how to check build definition history: