Probes that are intended to land on a planet are decontaminated prior to launch, orbiters which aren’t supposed to are not because it is an extensive and expensive process
For the Cassini-Huygens mission, the Huygens lander was decontaminated to avoid bringing any microbes to Titan. The much larger Cassini orbiter was not because it wasn’t supposed to go anywhere that any microbes on board would matter.
Since Cassini was not decontaminated it meant that it had to be disposed of after its primary mission in a way that ensured that the microbes on it wouldn’t survive and contaminate something that we might want to check for life later.
There are international guidelines on preventing forward and back contamination of planets that NASA and other space agencies follow
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