I just downloaded and compiled the data logger example from the DS3231_Simple library with Teensy LC selected. It compiled without issue and did not receive as messages about "Legacy" libraries in use. If I select the Pro/Pro Mini it compiles fine as well. I am using Arduino IDE 1.8.3 with Teensyduino 1.37-beta4 installed.
I have sometimes seen Legacy in parentheses near library names which indicate the library is not following the current Arduino library format but it will still compile and run.