Bash with a #!/bin/bash shebang. Typical GNU cultural imperialism. "Everyone has a Bash and it's always kept in /bin."

And the alternative when you definitely want bash because you've used extended non-posix features /bin/sh is not guaranteed to have?

Without just using #!/bin/sh (or some trick with env that may also not be portable), then having a large fancy function to test for current shell identity and tries to search for & respawn with something else if that does not seem to be suitably compatible?

The project has bash on its name - it is not as if it is pretending to be more generic, or written by someone who has always had bash linked from /bin/sh and knows no better.

Presumably:

    #!/usr/bin/env bash

#! env bash at least makes it path independent. But I think the parent comment is in jest.

Not enough useless whitespace for your taste?