The Sierra Negra volcano on Isabela is active. You can find here general information about the Sierra Negra volcano. The seismic activities are a lot higher and therefore the Galapagos National Park has prohibited tourists to get up to Volcan Chico. All Sierra Negra tours still go but one take about 2 hours instead of 4-5 hours. You can read about the increased seismicity in this article.
Whenever the complete hike is allowed again it will be updated on the website.