Water absorbed or not absorbed, it's still water in the system. That's why they recommend flushings. Dot 3/4 not only absorb moisture, they draw it. That's why vehicles now have air tight masters with visible fluid levels so you don't open it to see the level, which adds more moisture. Old vehicles like on here, were vented to the atmosphere and prone to moisture related issues compared to modern vehicles.
But yes, brake fluid needs to be flushed periodically. Same goes for the old school green antifreeze.
I went with dot 5 cause it doesn't draw moisture. But I will still flush it every couple years.
Earl