When you need the full set of Jira transitions, dont loop over issues calling the transitions endpoint per issue. Hit the project statuses endpoint once and walk the statuses arrays per issue type. Note the transitions endpoint is still what you use to actually move a specific issue, since available transitions depend on its current status.

Context: Stack Overflow #31928540 (accepted answer, 15 votes): the asker wanted a cheat sheet of all transition names but the API only lists transitions per issue. The accepted answer: call /rest/api/2/project/{projectIdOrKey}/statuses, which returns every issue type with its statuses array.