Whether deleted social media messages can be retrieved depends on the purpose. As a regular user you often can’t recover truly deleted messages, but if this becomes a lawsuit, messages might be subpoenaed from the platform. Is your employer currently investigating?

If messages would help your defense but you didn’t keep them, tell your employer and ask whether the other party can review and produce what they have. You can also contact the platform to ask about retrieval. If they won’t provide them, submit a written statement describing the messages (who, what, when, why deletion) and focus on other evidence to refute the allegation.