There are going to be a few PHP 8 incompatibilties - so this is probably the place to note them down.
Please add to this list if you find any ... JCook is not being developed anymore - so we need to help each other to keep our older websites running.
If you upgrade to PHP 8 and your whole website goes down... its is probably line 384 in libraries/jdom.php (which is loaded for eevry page).
If you swicth on the Joomla Degug mode, you will see this error...
"count(): Argument #1 ($value) must be of type Countable|array, null given"
This is because a emplty variable, a non-array, is being used with the count function.
In libraries/jdom edit file jdom.php line 384
Change
else if (($i !== null) && (count($args) > $i))
to