To accommodate the addition of a new highlighted server (Problem Solvers), we're testing a new cycling system for the highlighted servers. This means that out of the five servers currently on the highlighted list, only three will be highlighted at any given time, with the three chosen being switched out on a regular basis.
Right now, we're testing this system with three servers highlighted, changed per week (Monday at midnight); this may change as we find the ideal parameters to ensure maximum coverage of highlighted and non-highlighted servers alike.
And yes, all highlighted servers can still be played on. They may not be highlighted at this very instant, but they're definitely still there. If you'd like to keep track of them, a good way is to find them in the server browser, and then press Add to Favorites.