Seclists.org is a security mailing list archive, not a store or a service that asks for your personal data. It's run by the same people behind Nmap and Insecure.org, which gives it a strong pedigree in the security community. The domain has been active for over two decades, and the site has never been flagged for malware or scams. That's exactly what you'd expect from a legitimate community resource.
If you're here to browse security mailing list archives, you're not taking any real risk. The site uses a valid SSL certificate, so your connection is encrypted, and the only payment mention (bitcoin) likely relates to donations for the broader Insecure.org project. There's no login system, no checkout, no sensitive data being collected. The absence of modern security headers or the acceptance of older TLS versions is a technical nitpick, not a safety issue for a read-only archive. In short, seclists.org is what it claims to be: a dependable, well-known hub for security mailing list content.