Local authorities have advised that from 24 March, people seeking to enter El Salvador must be able to show, including to the relevant airline or travel company, that one of the following COVID-19 requirements is met:
That you have been vaccinated against COVID-19, including both doses if appropriate for that vaccine make.
Children under the age of 2 are exempt from presenting a COVID-19 test.
Travelers with proof of recovery from the virus within the last 90 days prior to boarding. Proof of a recent positive viral test and a letter from your healthcare provider or a public health official stating that you were cleared to travel.
More info at the CDC site
Travelers who did not get tested, are advised to stay home and self-quarantine for 10 days after travel.