Ubuntu's installation media (ISO) generator recently stopped providing assembly of source code ISOs. These ISOs contain all source code packages for Ubuntu Linux, and their original intention is to help users comply with the GPL license and ensure that the code is easily accessible. But it's of limited use in practice, and now Ubuntu developers are considering discontinuing the source code ISO.
Source code ISOs can in theory help ensure that the GPL licensed code being used is actually usable, but in practice they are not widely used... Think about the last time you downloaded six DVDs worth of Ubuntu source packages?
Individual source packages for Ubuntu/Debian are still provided in non-ISO form for individual use by users who require different assets. The Ubuntu source ISO is not mirrored with other Ubuntu ISO binary releases, so availability is lower.
It has also been pointed out that various Ubuntu desktop environments/hybrid versions have not released their corresponding source code ISOs. In addition, this only represents the status when the source code is released. If you want to obtain all current Ubuntu source code packages and security updates, you need to manually obtain the source code packages from the mirror.
Today, discussions finally started on the mailing list about stopping providing Ubuntu source ISOs altogether. Ubuntu source code will still be available, this is not to say that access to the source code is closed, but to consider whether it is worth the cost and ongoing maintenance burden of assembling a source code ISO.
It has also been suggested that Snap packages are not currently part of Ubuntu source ISO builds anyway, so these source ISOs are incomplete from start to finish and have been so for years.