A lot of phpbb boards were retired due to security concerns. They were rarely updated and often didn’t use https. Systems like that are easy targets for comment spam scripts as they are uniform and have broad reach. They don’t monetize well, as user-generated content can be unappealing to advertisers and tends to be categorically blacklisted. Overall they were and are more of a liability than asset to many sites. Still, I hope they stay a vibrant part of the web. Discourse is the go-to platform for new forums IMHO.