Jack and Aaron are right that the comment cannot be brought back, and Aaron's point about permissions is the part worth acting on today: narrowing Delete All Comments, and Delete Own Comments if your process can live without it, is the only real protection Jira gives you here.
One correction on the changelog, because anyone finding this thread later will try it and lose an afternoon. /rest/api/3/issue/{issueIdOrKey}/changelog returns changes to issue fields. Comment bodies are not in it — an edited or deleted comment leaves no entry there at all, neither the text nor who removed it. It is the same reason {{changelog.comment.fromString}} comes back empty in Automation.
Two sources so you do not have to take my word for it. Nic Brough put it plainly in another thread: "Your research is correct, Jira does not have a comment-edit history on the issues." And Atlassian's own open suggestion JRACLOUD-94821 says the History tab "does not record any information when a comment is edited or deleted, nor does it indicate who made these changes." If the changelog carried it, that request would not still be open.
For this specific ticket, two things that sometimes do work. If anyone was receiving notifications on that issue, the original comment notification email contains the comment text — that copy sits in people's inboxes rather than in Jira, and searching there is the most common way a deleted comment actually gets recovered. And if the comment arrived by email into a service project, the original message may still be in the mailbox the request was created from.
Going forward, the only way to have the previous text is to have been capturing it before the deletion happened. Disclosure: we make one of the apps that do this, so weigh the rest accordingly. There are three on Cloud — SaaSJet's Issue History for Jira, which Yuliia mentions above; codefortynine's Comment History Log; and ours, Comment Audit Trail. They share the same limitation and it is the one that matters: all three record from the day you install them, and none can recover anything from before that. Nothing on the market undoes what happened here.
If you do evaluate them, the question I would put to each vendor is whether anyone — including a site administrator — can delete a stored record. That is what decides whether the trail survives being inconvenient.
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