Are the referees of a journal allowed to reveal the title of the papers they review after the review process?
Don't do that.

Everything related to reviewing is confidential.

You will certainly upset some editors if you reveal your identity to the authors without the editor's permission. (Remember that the authors usually know who was the editor. Perhaps the editors did not want the authors to know that they asked you to review the paper?)

More generally, there is nothing to gain by doing this, and everything (= your reputation) to lose. You do not want to do anything that someone might interpret as a violation of the confidentiality of the peer-review process. (Even if you had both the authors' and the editor's permission to publish this information, others might not know that.)