You can pretty easily customize the advanced custom fields WYSIWYG toolbars globally, however doing so on a case-by-case basis needs to be done in JS since the editing experience is so dynamic.

This code is looking for a specific ACF field called step_text and sets the WYSIWYG editor to only allow bold, italic and links.

add_action(
    'acf/input/admin_footer',
    static function () {
        ?>
        <script type="text/javascript">
            (function ( $ ) {

                acf
                    .add_filter(
                        'wysiwyg_tinymce_settings',
                        function( mceInit, id, field ) {
                            if ( field.context.dataset.hasOwnProperty( 'type' ) && field.context.dataset.hasOwnProperty( 'name' ) && 'step_text' === field.context.dataset.name && 'wysiwyg' === field.context.dataset.type ) {
                                console.dir(mceInit.toolbar1);
                                mceInit.toolbar1 = 'bold,italic,link';
                            }
                            return mceInit;
                        }
                    )
                ;

            }
            )( jQuery );
        </script>
        <?php
    }
);

Update – 2021-02-12

This doesn’t seem to work universally. While working on another site, field.context was not being sent and instead, field appears to be (possibly) array-like. This fix was to use some different properties, and to also use a field key instead of name. Less optimal, but still not terrible.

add_action(
    'acf/input/admin_footer',
    static function () {
        ?>
        <script type="text/javascript">
            (function ($) {

                    acf
                        .add_filter(
                            'wysiwyg_tinymce_settings',
                            function (mceInit, id, field) {

                                if (field.data && typeof field.data === 'function') {
                                    const key = field.data().key;

                                    // Accreditation
                                    if (key === 'field_5d07ad30d9a98') {
                                        mceInit.toolbar1 = 'bold,italic,link';
                                    }
                                }
                                return mceInit;
                            }
                        )
                    ;

                }
            )(jQuery);
        </script>
        <?php
    }
);