A service supervisor reports that your process ended with exit code 143. What does that most likely mean?

Exit code 143 means the process was ended by SIGTERM. Shells and most supervisors report "killed by signal n" as 128 + n. SIGTERM is signal 15, so 128 + 15 = 143.

"The process ran out of memory" is wrong: the OOM (out-of-memory) killer sends SIGKILL, which shows as 137 (128 + 9). A program could call exit(143) on purpose. In practice, though, 143 means the process got SIGTERM and had no handler to exit in its own way.