SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
WHERE 
  cscart_products_categories.product_id IN (
    43255, 43254, 43253, 43252, 43251, 43250, 
    43249, 42769, 42768, 42767, 42766, 
    42765, 42764, 42763, 42762, 42761, 
    42759, 42758, 42757, 42756
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00040

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories range PRIMARY,pt pt 3 21 Using where; Using index
1 SIMPLE cscart_categories eq_ref PRIMARY,c_status,p_category_id PRIMARY 3 toolboxdigitalshop_dev.cscart_products_categories.category_id 1 Using where

Result

product_id category_ids
42756 1M
42757 3M
42758 1M
42759 3M
42761 3M
42762 3M
42763 1M
42764 4M
42765 4M
42766 4M
42767 4M
42768 4M
42769 17,13M
43249 22M
43250 22M
43251 22M
43252 22M
43253 22M
43254 22M
43255 22M