It's still very much a thing. I've posted a couple of examples directly above (DTW-SFO and DTW-AMS have fares with roundtrip booking and Saturday night stay requirements on DL). They are less prevalent domestically due to competition from airlines like WN and AS who do not have Saturday stay fares at all and everything is one-way's (and the legacy 3 tend to match their fares on non-stop routes where they also have flights). For TATL fares, they are still commonplace. Airports that don't have much non-stop competition from WN and AS and other LCC's tend to have more of them domestic routes. The fare differences tend to be less dramatic domestically. As I posted above, the difference between the cheapest DL roundtrip fares on DTW-SFO with a Saturday night stay and without is $359 vs. $497 roundtrip.