Thanks @John-Pierre Cornelissen
Yoast support has kindly answered that they "can't give personal support for our free plugins" (and i understand this).
Just as additional information, i dug by myself in the plugin folder, and i found out that these two files:
- frontend/class-universal.php (line 165 and subsequent)
- frontend/vews/tracking-universal.php
are both written with the new "__gaTracks" code, in accordance with the google guideline, as you properly reported.