Assume we need the same fix in com_content here: https://github.com/joomla/joomla-cms/blob/staging/components/com_content/views/categories/tmpl/default_items.php#L31 ?

No, that's good.

and in com_newsfeeds too is OK.

Categories all have the same param names though. So why is com_content different to everything else?

So com_newsfeeds and com_contact use one param and com_content and com_weblinks (https://github.com/joomla-extensions/weblinks/blob/master/src/com_weblinks/site/views/categories/tmpl/default_items.php#L31) use a different one?? Something seems really odd here....

those are the parameters to turn on/off the displaying of items count inside each category when you list all categories. It seems someone some time ago did an odd copy/paste/modify job...

com_weblink I didn't tested... let me 5 minutes to see...

com_weblink is wrong: it should be show_cat_num_links_cat

Basically each components has his own parameter name (why?) and the name for com_contact is anomalous because it doesn't have 'num_contact' but 'items'. The '_cat' at the end of the names is a mystery only the original coder can explain...

and yes, sorry, com_newsfeeds too has 'items', but it is the same in the .xml file, so it is OK

Initially it was show_item_count for all, but then... ooops... bfc18c6

joomla-extensions/weblinks#22 for com_weblinks

Beez was OK.
Obvious error. Merging. Thanks.