Liferay provides some site templates out of the box with the goal of showcasing the site template functionality. The original goal was for this site templates to also provide a good starting point to cover real world use cases. However, because they haven't been adapted with the years to these real world needs they currently add very little value. The same applies to the out of the box page templates.
Considering this it's been decided that it is better to remove the out of the box site templates and page templates so that they are not created in new installations. Existing installations will not be affected by this change, since the site/page templates will already have been created and thus will not be impacted.
Note: It's worth noting that there are also independent efforts to provide Accelerators or ready to be used Solutions for Liferay DXP that will be a much more complete solution for the need that these site initializers attempted to solve in the first place.