Is this still a work in progress? Because I think you'll need a little more to make this feature work.

This is technically correct, but not the best solution :)
On the call site union('SELECT * FROM table', FALSE, TRUE) doesn't tell you much. But even factoring in named parameters, two arguments still feels odd for alternative options.
I.e. union('SELECT * FROM table', TRUE, TRUE) makes no sense.
So it's better to break the API and change the second parameter to be more inclusive :)

whitespace error :)

There should probably be a test for the old value too (boolean).

These can be collapsed into a "UNION $type"

missing break

You copy-pasted this, and this isn't wrong, so just consider this an FYI :)
In newer code we do
->with('query') instead of ->with($this->equalTo('query')
and
->willReturn('(query)') instead of ->will($this->returnValue('(query)'))

perhaps NULL is a better default value?

This is good, but it should be in the sql_compound(). Otherwise we have to repeat this code for other compound query types like EXCEPT or INTERSECT.
(Hint: That's what you'll work on next ;) )

I'd just make it a separate parameter. No need to make things more complicated then they need to be :)

style ;)

Minor nitpick. We don't need the rtrim here, as the string passed here is always static and never user input.

Branch information is missing :)

Diff contains unrelated changes :)

It's better if you split $types into 2 separate variables here (already at the input) and adjust the data provider accordingly.

We also now need a test for unsupported operators

All these tests can be combined, with a data provider giving the individual test cases.

Probably good to run phpcs on this ;)

Totally nitpicking here :)

/**
 * Test union()
 *
 * @param ...
 *
 * @dataProvider unionOperatorProvider
 * @covers       Lunr\...
 */
public function testUnion($operator): void


So, missing @param, correct alignment as spacing in the phpdoc, and singular param name.

values should be aligned

spaces after the comma