A project I'm working on has 2 long-standing feature branches as well as the master branch.
To fully automate deployments, I'd like to create a pull request from master into those two feature branches anytime a deployment goes out from a VSTS Release.
What kind of tooling in VSTS would allow me to do create pull requests as a release task?
You can create the Pull Request through Pull Request REST API during the release.
There is Invoke HTTP REST API task but may not good for your requirement.
The simple way is that you can do it through PowerShell task:
Simple sample:
Arguments:
-project "XXX" -repo "XXX" -sourceBranch "XX" -targetBranch "XX" -title "XX" -des "XX" -token [$(System.AccessToken) or personal access token]