In the same di.xml file I found that lessFileSourceBase is passed as an argument for lessFileSourceBaseFiltered that is passed as an argument for lessFileSourceBaseSorted that is passed as an argument for type Magento\Framework\Less\File\Collector\Aggregated.

I found no other occurrence of lessFileSourceBase (or lessFileSource ) in an other file except di.xml from the core module. Only in some cache files but those are not important.

I guess if you are not going to use the virtual type in a PHP class, but only in the di xml files then you are not required to make it look like a class name and you can use an alias.

But this is just pure speculation.
It will be fun to try to create a class and inject in its constructor an instance of lessFileSourceBase to see how it behaves.

In the same di.xml file I found that lessFileSourceBase is passed as an argument for lessFileSourceBaseFiltered that is passed as an argument for lessFileSourceBaseSorted that is passed as an argument for type Magento\Framework\Less\File\Collector\Aggregated.

I found no other occurrence of lessFileSourceBase (or lessFileSource ) in an other file except di.xml from the core module. Only in some cache files but those are not important.

I guess if you are not going to use the virtual type in a PHP class, but only in the di xml files then you are not required to make it look like a class name and you can use an alias.

But this is just pure speculation.
It will be "fun" to try to create a class and inject in its constructor an instance of lessFileSourceBase to see how it behaves.