LFS is not a traditional distro, but a book telling people "how to". So the released book won't be updated for security fixes.
Now, for items since LFS-10.0 (and BLFS-10.0) were released on 1st September 2020, advisories are (sometimes belatedly for 10.0 items) being documented. There will be separate pages for each release of the book (covering up to the next release) and there is also a consolidated page for all LFS and BLFS security advisories. The advisories are avaliable here.
Pre-releases (x.y-rcz) do not have security advisories. Users building
them should take care on the changelog to identify security issues.
For security issues in a package used by LFS book, report them to upstream developers directly. Do not disclose any security issue without upstream approval.
For security issues in the LFS book itself (for example, using an outdated package with security vulnerabilities, building a package without a critical security feature recommended by upstream, or creating a vulnerable system configuration), report to the editors and maintainers listed in here. It's recommended not to disclose a security issue without approval from the managing editor.
For security issues introduced by a mistranslation in this repo (unlikely but it may happen), report to xry111@xry111.site.