I created a SlackBuild for syslinux 4.05 but slackbuilds.org does not accept SlackBuilds that are upgrades of the standard Slackware packages. Fair enough.
More relevantly to your post, the slackbuilds.org mailing list had this in response from Rob McGee:
I remember discussing this with Pat some years back. The details are
forgotten, and unfortunately I don't have the archive of the
discussion, which became a three-way with HPA, the syslinux author.
That part of the discussion did NOT go well, I do recall. Pat's
conclusion was that the older version did what Slackware needed, so
upgrading was not worth the bother.
I suggest that you take up this issue with Pat. Maybe you have found
solutions for the problems he had with newer syslinux versions?
I have only tested the EXTLINUX component and that not extensively.