I tend to think that older sayings and words don't sound quite so old-fashioned, but "swell" strikes me as quite dated.
That said, a quick search shows some hits, so it is used.
"On the other hand we are fully open to the possibility that it could and it might, and if it did we will concede to Mother Nature and step back and applaud and be grateful for the fact that California would get that kind of relief. If El Niño comes, that will be swell."
A group called Box of Birds released a song in 2015 titled "It's Fine, It's Swell," but I would tend to think it belongs more in the era of Duke Ellington's 1937 song "It's Swell of You."