The Fog of Ages: This didn’t so much happen to the natural immortals of the setting, but then, the natural immortals weren’t all that natural, having been designed into immortality by one particular group of Precursors, and it is generally believed that a few memory upgrades came with the deal.  While it probably would have eventually – well, these days, they’re using offloading extra memory and cognitive processing to the cloud and its ultratech nanocomputronium cores, so it’s unlikely to ever come up in the future.

It can, which is relevant where immortalist proselytizing comes into play, affect people who’ve just taken an immortagen and nothing else, but, well, if it bothers you, they also have this nice catalog full of brain upgrades, memory-management techniques and advertisements for memory-redaction and psychedesign services right here, and they’d be happy to sell you some.