Get category id

Try this in your block file and phtml file

$this->getData("category_id");

In Magento 2 You can get Category Id by using following code

$categoryid = $block->getData('category_id');

I hope this will help

At Block, you can get that parameter value by

    if($this->hasData('category_id'))
    {
        $categoryid = $this->getData('category_id');
    }