Secretary of State George P. Shultz's controversial plan to make L. Craig Johnstone, one of his key aides in Central America policy-making, ambassador to Honduras has been shelved, State Department officials said yesterday.

Conservatives, who have charged Shultz with trying to purge the department of political appointees and others who share their hard-line views, had opposed Johnstone, a deputy assistant secretary for inter-American affairs, on grounds that he has not shown sufficient vigor in combating activities of Nicaragua's Sandinista government.

However, senior department officials said Johnstone's proposed appointment to Honduras had been withdrawn not because of conservative pressure but because he did not want to take the assignment, for personal reasons. As a result, one official said, there was "the chance for a marriage of convenience that satisfied Craig's personal wishes and that gave the conservatives an opportunity to declare a victory."

Whatever the reason, the decision was an about-face from a week ago when officials said Shultz and his assistant secretary for inter-American affairs, Langhorne A. (Tony) Motley, had cleared Johnstone's appointment with President Reagan and were prepared to stick with it even at the cost of a battle with Senate conservatives threatening to oppose his confirmation.