Jewish emigration from the Soviet Union continues a slow, steady decline this year from the record level of 1979 when the SALT II treaty was signed and the Kremlin still had grounds to hope for new American trade concessions.

Although there is apprehension abroad that the slackening pace of permissions may eventually bring to a halt significant emigration from the Soviet Union, knowledgeable sources here say it is simply too soon to predict overall Soviet intentions or policy on the controversial question.

In all, about 10,500 Jews left the country through the first four months of 1980. Although this indicates an average of 2,600 a month, the actual totals have dropped steadily by about 500 per month from January's 3,300 to April's 2,000.

More than 200,000 Jews left the Soviet Union in the past decade, when detente dominated Soviet relations and the United States seemed inclined to grant special trade status to the Soviet Union in return for increased Jewish emigration. More than 50,000 Jews were permitted to leave last year, but the cooler international climate and Afghanistan invasion shelved most-favored-nation trade status for Russia, although China, Moscow's archrival, gained it.

Jewish emigration from the principal Ukrainian cities of Kiev, Odessa and Kharkov has all but ceased, sources said, and they noted a sudden sharp drop in Moscow permissions last month as well. Some here think this may be a sign the Soviets intend to "save" Moscow's quota of emigrants for the time of the Summer Olympic Games here, when the city will be virtually closed to Soviet non-residents, making it impossible for privincial Jews to have their exit permissions fully processed.

But even that is speculation.Visa officials of the Ministry of Internal Affairs do not discuss their policies.

Meanwhile, the number of Jews refused permission to leave reportedly continues to climb. Some sources have said they believe the Soviets may be inadvertently creating a new and growing group of disgruntled citizens, since otkazniki ("refusedniks") normally are fired from their jobs and frequently are harassed by security police. It is thought about 300,000 Jews have received invitations to emigrate from relatives abroad, and that perhaps 100,000 are actively interested in leaving.