Rumors suggest that this was originally thought up as the series finale if the show was not going to be renewed for further seasons. And judging from this episode, it's very easy to believe that statement as the episode goes back to a lot of elements that were found very early in the show, and it deals with certain loose plot points that needed some answers. It feels like a return-to-form for the show and it represents a show without a clear idea of its future. And the episode itself also represents the season as a whole, which has been fairly good, but not great.
It's easy to see how far the show has evolved through this season. The small scale of the first season is largely gone and it's quite interesting to see how those early days and the characters still fit into this larger universe where a lot of the puzzles have been moved into place. However, it's also easy to see how far the season alone has gone as it seems to have gone back to a lot of its original themes and ideas, leaving behind the story of the aliens' origins on Earth. It's both a big help for someone who may think that the show has gone too far and is going above what's best for it, but it's certainly also a detriment as a lot of those plotlines have been left largely unsolved. However, the episode still touches on some very important things that have happened throughout, with a lot of regular friends showing up in this episode and getting the opportunity to cap off their story in a semi-satisfying way, although the resolution of the Smoking Man's arc does leave a lot to be desired. It leaves the show off on an uncertain note, finding great joy in leaving everybody unsure about what is going to come after and how the future will shape up. These characters did come back for more seasons, but it probably shaped the show in a different light than it had until then and that will be interesting to see when I start it up again in a few weeks.
"Requiem" does feel like it's the end of an era of "X-Files," while still keeping the door open for the new days to shine through. It goes back to the beginning and brings a lot of familiar faces back, using them all to tell a satisfying story that wraps up a lot, although not everything.