One of the method of accessing older threads is clicking one of the PSoC series product (PSoC 5LP MCU as example) as below:
Then click the "Content" tab as below, use the "Filter by tag" to sorting all the PSoC 5LP MCU threads and use the "arrow" at the top right corner to find older threads.
Thanks for the hint, but it would have been nice to have the "More" button actually working. Or the Content tab working (currently empty). I know that some improvements has been done to the forums recently, but many bugs were introduced as well. For example, notification doesn't work any more, score counter doesn't work, and private communication doesn't show notification.