Is the Trojan virus message on my iPhone a legitimate warning?

You simply visited a less than legitimate website that was paid to bring up that page to you. You just need to close it and move along. Most legitimate websites do not rely on these predatory advertisements for money when they have their users best interest in mind for the content they want to provide to them. It is your choice what sites you visit and it is that sites choice what they want to display to you. Choose your destinations wisely.

Of course not: Phony "tech support" / "ransomware" popups and web pages - Apple Community

No. There isn't any such thing as a Trojan virus, and any other notification of malware in a web browser will only be legitimate if someone manually started a scanner(only possible on Windows) or uploaded a file.

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