Here's the HN discussion for that article that puts it into context: https://news.ycombinator.com/item?id=33651693
The author is only speculating about Mastodon's search feature, and I see no actual evidence that the search feature is intentionally limited due to child sexual abuse material.
The one instance I have any prior awareness of, the mastodon creator saying search is not desired due to negative social dynamics, everywhere else it was presented as a privacy and anonymity behaviour - crucially, both from those who agreed and disagreed (which makes intuitive sense; on one hand I dislike e.g. Facebook not being publicly searchable some of the time, at the same time I don't want my content crawled by randos all the time either). This is the first place I've seen that frames it as explicitly CSAM related.