TYPO3 Core

Upgrading from older version the former makeCategorizable() method allowed to limit the category selection to a single item. (the MM relation was the only supported way)

The new TCA type "category" currently disallows maxItems=1 for anything except oneToOne.
This makes switching to the new TCA type impossible for upgrades, unless one migrates all data away.

There is no technical reason, why a manyToMany relation should not be restricted to 1, specifically as any other number is allowed.

TCA looks like this

        'categories' => [
            'exclude' => true,
            'label' => 'LLL:EXT:core/Resources/Private/Language/locallang_tca.xlf:sys_category.categories',
            'config' => [
                'type' => 'category',
                'size' => 10,
                'maxitems' => 1,
            ],
        ],