Ok, so this is a move of desperation. Due to the onslaught of misbehaving AI bots, I've been forced to require login even for read access to most forum sections.
This is nothing I wanted, but the forum was inacessible for long periods due to the constant DDOS from the scrapers. We're talking a constant traffic of 10-20 hits per second, which caused mysqld to have an average load of above 95% cpu.
If they had respected robots.txt, this would not need to happen. But they don't.