IMSI's web page listing file import/export capabilities for TurboCAD Windows hasn't been updated to 2019, yet (get off the pot and do that, okay?), so I can't confirm that STL import went away in this version.
However, if you can upload the file, one of us can convert it to TCW.
Jeff
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TC Pro Platinum 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1