Try bellow code 


    <?php
    
    namespace Mainf\Stickers\Block;
    
    class Discount extends \Magento\Framework\View\Element\Template
    {
        protected $_registry;
    
        public function __construct(
            \Magento\Backend\Block\Template\Context $context,       
            \Magento\Framework\Registry $registry,
            array $data = []
        )
        {       
            $this->_registry = $registry;
            parent::__construct($context, $data);
        }
    
        public function _prepareLayout()
        {
            return parent::_prepareLayout();
        }
    
        public function getCurrentCategory()
        {       
            return $this->_registry->registry('current_category');
        }
    
        public function getCurrentProduct()
        {       
            return $this->_registry->registry('current_product');
        }   
    
    }
    ?>


Display current product data in your template (custom.phtml) file

    if ($currentProduct = $block->getCurrentProduct()) {
        echo $currentProduct->getName() . '<br />';
        echo $currentProduct->getSku() . '<br />';
        echo $currentProduct->getId() . '<br />';       
    }