Gee, it does appear to be the case. I first searched for all of my posts ever. I got 35 pages worth, going back to 2006. Great, I thought. But then I took one of the earliest posts, with the phrase "Thalkirchen Kletterzentrum" in the subject line.
Searching for that revealed no results. So it appears that older posts still exist, but are not indexed in the search engine.
It's a shame, because newbies are often encouraged to search the forum before posting a "dumb" or "old" question here. Indeed, a fully indexed forum here is a valuable and even important resource for climbers. But the index is apparently truncated to 1 year. Is that really necessary for something like site performance or space savings?