Yes, it's true, also for bigstockphoto, Featurepics, and others, although not always six months for all sites. Some of them require only three months.

Dreamstime will not allow you do delete any photos before six months, and only deactivate 30 percent of you photos with less than six months. And, considering they have one of the worst watermarks in the market, be very aware of that, before uploading some very good photos that would be instantly stolen.

For example, their watermark doesn't show at all on B/W photos. I have one of mine with hundreds of views and no downloads... :-)