I'm not sure what makes this a specifically startup marketing plan. This a very traditional marketing plan template. In fact, I would argue that a startup in the pre product-market fit stage of development would be advised against sticking to a plan like this, since things like positioning, target audience and strategies could change month-to-month and even day-to-day.
But if you were following this traditional marketing plan, I still think there's a very important section missing and that's measurement and testing. This is one of the more important parts of your marketing plan. You should devise your tests with thought and purpose behind them and not as an afterthought.
I don't see what's traditional in this plan. It takes an hour to fill, 10 minutes to update. It's not a huge commitment, and you can always change direction, but in the meantime everyone on the team is on the same page.
A lot of founders just don't know how to get started with marketing. Now I can link to this. This a better time investment to fill this out than to read half of the so called marketing advice you can find online.
I called it a startup marketing plan, because I've used it for six years while working with startups. It's not groundbreaking, but it's way less overwhelming than the comprehensive marketing plan I used for larger companies.
I completely agree with testing and revising on an ongoing basis. Of course you've got to continuously test assumptions, measure results, and adjust any plan.
But if you were following this traditional marketing plan, I still think there's a very important section missing and that's measurement and testing. This is one of the more important parts of your marketing plan. You should devise your tests with thought and purpose behind them and not as an afterthought.