if you are speaking of Mexico City, Mass was celebrated at the cathedral but the public was not allowed in and were turned away. the archbishop dispensed Catholics from their Mass obligation, but not priests from the obligation to celebrate Mass, which presumably they did in empty churches and chapels.
Catholics automatically are dispensed from the obligation to attend Mass when their health does not permit it. By way of analogy, when a major epidemic would potentially threaten the health of all those attending Mass, it seems appropriate for the Bishop to dispense people from the obligation to attend (and indeed to prevent people from attending as seems to be the case here). I’m not sure of a direct precedent. It would be interesting to know if something similar was ever done during the 1918 flu pandemic or other similar outbreaks.
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