Posts "aren't highlighted after 6 months" means that Google limits the time a post is highlighted across its products. At one point posts were visible for only 7 days, now the intent is to display posts for 6 months. I received a marketing email encouraging a new post at the 6-month date, however, my post is still visible even a few days later. To note, there is an archive link that shows older posts. This archive listing is accessible from the VIEW ALL button. Therefore, essentially "highlighted" means showing up on Google Maps and the Knowledge graph as a post without having to click VIEW ALL to be found.