“mvn dependency:tree” – is there an equivalent available for “verbose” output?

You can use

mvn dependency:tree -X

which produces the debug output.

Otherwise stated in the documentation about verbose –

Notice this feature actually uses Maven 2 algorithm and may give wrong
results when used with Maven 3.

Edit: As pointed by Brad in the comments, the verbose flag has been reintroduced for the 3.2.0 release and further.