I can only tell from my experience:
It is outdated the minute you finish writing it. I hated test-plans since day 1 of my career. I successfully avoided test-plans for the past 6 years. I quit my last job when I had to work with a team that placed test-plans above all else.
Test plans are a burden that no one cares about, except for QA managers and the QA people who write and maintain them. The bigger the project the more test-plans there are to maintain. Eliminate QA mangers and the test-plans will die off.
QA mangers love test plans as it lets them “manage” people, “estimate” project scope, draw graphs – at the same time they can be clueless on what is going on in the project.
Be competent in what you do. Don’t hide behind test plans whenever there are issue uncovered in production.
Most of us are working in agile environment and test-plans are incompatable with it.
My suggestion – use checklists – its fast, informal, open to interpretation doesn’t loose value as fast as test-plans as it doesn’t provide you steps or expected results – only general idea that needs to be tested.