Personally I’m happy to ask them to kindly maintain my new feature along with all of their other code, so I can continue to use an up-to-date version with very little effort.
And in reality it depends a lot on my level of contribution. If I have dozens of lines of code in a big project, then who cares, all I really wanted was for my pet bugs to get fixed.
If I contributed major features or thousands of lines of code there, then I would be very mad. But I would also be very capable of maintaining an open source fork of the project.