It doesn't sound reasonable to me tbh. If anything, reading those manifestos is a good way to learn just how nutty those people are in the first place. At the same time, having it accessible prevents speculation about motives, which can lead to false justification for politically oppressive measures.

OTOH if the goal is to prevent copycats then I don't see the point of a 90-day embargo. People who are likely to take that kind of content seriously enough to emulate are still going to do so. Tarrant, for example, specifically referenced Anders Breivik.