The older GTX prefix is now used to denote older Nvidia graphics cards which don't have the extra AI and ray tracing silicon that the RTX-level cards do. This RTX prefix was introduced with the RTX 20-series, and highlights which cards have GPUs which sport both the Tensor Cores and RT Cores necessary for real-time ray tracing and Deep Learning Super Sampling (DLSS).
Nowadays you'll only find older 16-series GPUs with the GTX prefix attached, so it's pretty much RTX all the way.