Change 328601 had a related patch set uploaded (by Huji):
OOjs UI should allow disabled to contain an array

https://gerrit.wikimedia.org/r/328601

Using the patch above and code like this:

			'grants' => [
				'type' => 'multiselect',
				'options' => [ 'Alphas' => 'alpha', 'Bravos' => 'bravo', 'Charlies' => 'charlie' ],
				'default' => [ 'bravo', 'charlie' ],
				'disabled' => [ 'bravo' ],
			],

I get this output:

An alternative approach was proposed by @matmarex which would look like this:

new OOUI\CheckboxMultiselectInputWidget( [
				'value' => [ 'dog', 'cat' ],
				'options' => [
					[
						'data' => 'cat',
						'label' => 'Cat'
					],
					[
						'disabled' => true,
						'label' => 'Dog'
					],
					[
						'data' => 'goldfish',
						'label' => 'Goldfish'
					],
				]
			] )

However, that is not backward compatible (the options parameter is currently an associative array of type key => value and this approach will replace it with a non-associative array of associative arrays [key => val], [key => val]. Besides, the method I proposed is consistent with how the default option works (you just specify which options should be checked by default, and with my patch you also specify which options should be disabled).

So I am going to stick to my approach.

Change 328601 merged by jenkins-bot:
CheckboxMultiselectInputWidget: Allow disabling specific options

https://gerrit.wikimedia.org/r/328601