It seems like the second parameter has only been introduced in PHP 5.5.16, but Matomo supports (I think 5.5.9+)
We will need to detect the PHP version and change logic based on this. I would say in PHP.5.5.9-PHP 5.5.15 we can simply not include the ucwords and assume user is not using curl2? Could also just explode instead and run strtoupper and all elements and then implode again...
Disabling the fix in https://github.com/matomo-org/matomo/pull/14449 for the affected PHP versions sounds fine to me. No one apart from me even reported the original issue and as you say an ancient PHP version in combination with a really new curl version would be odd.