We ended up changing the approach: instead of telling ESBuild to ignore a file on watch mode, we do not write the file if its contents are the same, like so:

const newContent = (...)

if (fs.existsSync(path)) {
  const currentContent = fs.readFileSync(path, 'utf8');
  if (currentContent === newContent) {
    process.exit(0);
  }
}

fs.writeFileSync(path, newContent);