From what I can tell, about 30% of all gasoline sold in the US right now contains MTBE. It has not gone away.

MTBE is (probably) what degraded older rubber and plastic seals, leadeing to numerous carburator fires, even deadly crashes. Ethanol is much more benign.

MTBE did not replace lead. It was supposed to be an "oxygenate" to lower carbon monoxide emissions. Lead was removed for health reasons. The newer low compression engines and computer controls allowed the use of lower octane rated gas.