You can easily do it on your own.

In your custom plugin add the action path in your elgg-plugin.php:

return [
   'actions' => [
      'river/delete' => [],
   ],
];

Now create a file actions/river/delete.php with the following content

$id = (int) get_input('id');

$items = elgg_get_river(['id' => $id]);
if (!$items) {
	return elgg_error_response(elgg_echo('river:delete:fail'));
}

$item = $items[0];
$entity = get_entity($item->object_guid);

if (!$item->canDelete()) {
	return elgg_error_response(elgg_echo('river:delete:lack_permission'));
}

if (!$item->delete()) {
	return elgg_error_response(elgg_echo('river:delete:fail'));
}

$entity->delete();

return elgg_ok_response('', elgg_echo('river:delete:success'));

remember to flush your cache and run an upgrade before testing it.!

BE CAREFUL HERE!!!

Firstly, the delete option (of river items on the activity page) is only available to admins by default. If it would be intended for normal users also being able to delete river items (AND possibly deleting also the content the items belong to) is something that would have to be changed additionally.

But the main thing to consider is that not all river entries belong to Elgg entity OBJECTS (blogs, files, comments, etc.). Some entries are connected with USER or GROUP entities (e.g. a user updated his profile, joined a group, etc.). As long as only the river entry gets deleted this might not be too severe. But if you delete the entity (AS ADMIN!) that is the object entity of the river entry you would delete the user or group if you changed the code for not only the river entry getting deleted but the corresponding object entity too. So, you could permanently delete a user or group (together with all the user's / group's content) with one thoughtless click!!!

There is a high chance that your elgg 2.x plugin will not support elgg 3.0. There has been lot of changes in the core due to which the plugin might not work.

Here's the list of changes: https://github.com/Elgg/Elgg/blob/3.0/docs/appendix/upgrade-notes/2.x-to-3.0.rst

If your plugin is developed by someone else, then you can contact the person to update your plugin and make it compatible with 3.0. Or, if you have developed the plugin then you need to make the required changes as mentioned in the link above.

Regarding the error_log, can you check phpinfo() once, just to be sure that the path for your php error_log file is correct.

Thanks Rohit. There are several Elgg core functions we used in our plugins that are now deprecated in Elgg 3.0.*. I am working on replacing those deprecated functions with newer ones.

About the error_log, now I can see my error_log by turning on the logging in mysite/admin/settings/advanced