Can you propose a PR? See https://docs.joomla.org/Using_the_Github_UI_to_Make_Pull_Requests if you need assistance in how to create one.

@amazeika Is that related to #4308? Sounds at least like a similar issue.

If I know the solution, I would not commit the issue without code. The code looks ok [in ./administrator/components/com_plugins/models/plugin.php the parent:save from JModelAdmin is called and that save method has the triggers]... but I don't know why they are not used...

@Bakual Hello Thomas, these events are triggering just fine on my end when saving plugins. I've just reviewed the code and tried it myself.

I tried it, too - again.
Installation: Joomla! 3.4.3 Stable with Sample Data.

I added this:

public function onExtensionAfterSave($context, &$table, $isNew)
{
      var_dump($context);
      die(); // I know that die() isn't for debugging but it shows the best contrast (-> white site with a bit of text) 
}

to a standard core system plugin that is always activated: Logout (plugins/system/logout/logout.php) right after the constructor.
Its working in modules, templates, languages and on saving components options but not on plugins.

Removing the & from the $table in your code fixed it for me.
The issue comes from the fact that com_plugin is using the save method from JModelAdmin while the other managers have own save methods.
They differ in the trigger call in that JModelAdmin doesn't explicitely pass $table as reference while the others do.
Imho, the table is passed by reference anyway, so this isn't needed.

If you look at your PHP log, you will see a warning like this:

PHP Warning: Parameter 2 to PlgSystemLogout::onExtensionAfterSave() expected to be a reference, value given in D:\xampp\htdocs\joomla3.4\libraries\joomla\event\event.php on line 69

That it was! Sorry. log_errors show more errors on Joomla, as only display_errors...

For me the issue is closed. Thanks for your help.