Some types of RTV were labeled “Oxygen sensor safe” in the '80s and '90s. I think all automotive engine RTV is oxygen sensor safe now. There were many warnings about this back then.
Here is a Volkswagen bulletin;
Group: 26
Number: 93-02
Date: July 31, 1993
Subject:
Oxygen Sensor Contamination Due to Silicone
Model(s):
All Models, All Model Years
(Supercedes Technical Bulletin 92-01, Repair Group 26, November 30, 1992)
CAUTION
Do NOT use any sprays or compounds containing silicone on engines equipped with Oxygen Sensors. Do NOT use these compounds on or near the intake air system or near the Oxygen sensor. Silicone drawn into the intake air system is not burned during combustion and will lead to contamination and malfunctioning of the Oxygen Sensor.