There are a few principles that apply here. You can allow members to delete things that they think are no longer relevant. You can have only the most recent posts appear on the site, or you can have events appear in reverse chronological order. While member-generated content should be eternal, individual transgressions should have a statute of limitations. A successful community learns from its mistakes.
Every community has a shared sense of itself.
But that doesn't matter much if members don't know what's going on. If you went to a PTA meeting, you'd never have any questions about what was going on. People would either be talking about a topic, or they wouldn't be talking about it. Online, the points of interest aren't as immediately obvious. Communities need to provide a way for members to exchange things of value.