Why is the ~/ResourcePackages/Bootstrap4 resource package folder excluded from the project when upgrading the project to Sitefinity version 15.3.xxxx? 0.

After upgrading the project to Sitefinity version 15.3, the Bootstrap4 resource package folder gets excluded from the project.

As of Sitefinity 14.1, Sitefinity CMS ships with the Bootstrap 5 resource package only, and this is the officially supported Bootstrap version.

As of Sitefinity 15.3, the Bootstrap4 package is no longer distributed with the Telerik.Sitefinity.Feather NuGet package. As a result, these templates will no longer be included in the NuGet packages.

Previously, the Telerik.Sitefinity.Feather.nupkg package contained a folder "content\ResourcePackages" with both Bootstrap4 and Bootstrap5 templates.  Moving forward, only the Bootstrap5 version will be available.

Upgrading from a prior version of Sitefinity to 15.3 will remove the contents of the Bootstrap4 folder as well. This behavior comes from NuGet. When updating a package, first the old version is removed, which would include the files in the content directory, and then the new version of the package is installed with the new content directory.

If the content files have any changes before the upgrade, NuGet will keep them locally as it will recognize that they contain modifications. The rest of the files will be removed as there is no longer a Bootstrap4 folder in the new nupkg file to replace them with.

If upgrading using Visual Studio, the recommendation is to use source control so that any files that are removed during the upgrade can be restored if needed.
 
If upgrading using the Sitefinity CLI, there is a built-in logic that automatically backs up the resource packages that will preserve the Bootstrap4 folder.

After the upgrade is done successfully, re-include the Bootstrap4 folder or restore it from the backup if this folder is removed by the upgrade process. 

Backing up your project prior to starting the Sitefinity CMS upgrade is recommended, so that any files that were removed during the upgrade can be restored if required.

References to Other Documentation:

1. Sitefinity Documentation, Resource packages https://www.progress.com/documentation/sitefinity-cms/overview-resource-packages-mvc
2. Sitefinity Release Notes, Sitefinity CMS 15.3.8500 (Official Release) https://www.progress.com/sitefinity-cms/release-notes/sitefinity-cms-15-3-8500-(official-release)