2 classes are annotated with ViewsArgument

TaxonomyViewsArgumentTest in core/modules/taxonomy/tests/modules/taxonomy_test/src/Plugin/views/argument/TaxonomyViewsArgumentTest.php
Test argument handler for testing deprecation in IndexTidDepth plugin.
UserUid in core/modules/tracker/src/Plugin/views/argument/UserUid.php
UID argument to check for nodes that user posted or commented on.

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