civicrmHooks.php no longer invoked under Joomla and CiviCRM 4.7.20
I've spot-checked the current 4.7.21-rc and as well as an older version 4.7.15 -- the problem reproduces in both. The problem arises with web-based requests in which these two operations are performed in the wrong order: (a) updating the include_path and (b) loading the civicrmHooks.php from the include_path.

(Doing a proper audit of Joomla regressions is painful, but I'd wager the problem originates in the 4.7 alpha/beta cycle.)

Searching JIRA for civicrmHooks.php turns up a nearly identical issue CRM-19914, which was fixed in WordPress -- but it was incorrectly described/triaged as a "Windows" issue. (The bug actually would have applied to Joomla+WordPress, regardless of OS.)

I'll submit a similar patch for Joomla under CRM-19914.