Did that work in J3?
It surely works if you add the plugin group as in all other plugins: plg_content_nameoftheplugin.

Can't confirm this issue. I created a test plugin here: https://github.com/GHSVS-de/plg_mybaseplugin

As long as the mandatory attributes type and group are configured correctly in the mandatory manifest XML the name of the plugin can be whatever you want.

See https://github.com/GHSVS-de/plg_mybaseplugin/blob/main/mybaseplugin.xml#L2-L3

You can also use a custom group (= folder in /plugins/) if you want.

The demo plugin can be installed, activated, opened, saved and uninstalled without any issues:
https://github.com/GHSVS-de/plg_mybaseplugin/raw/main/plg_mybaseplugin.zip

Same here, also checked with capital letters. So please @ZhukDmitry specify what exactly does not work - error message? How did you install your plugin? you also can append your manifestfile here.

Difference between : plg_myBasePlugin and plg_MyPlugin
After the underscore = starts with a capital letter = don't know if relevant?

It works in J3, I just trying convert to support J4

Here my test plugin
-- manifest

<languages>
            <language tag="en-GB">language/en-GB/en-GB.plg_content_plg_zhosmmap.ini</language>
</languages>
    <!-- UPDATESERVER DEFINITION -->
    <updateservers>
            <!-- Note: No spaces or linebreaks allowed between the server tags -->
            <server type="extension" priority="1" name="Zh OSM Map Plugin Update Site">http://zhuk.cc/update/plg_zhosmmap/extension.xml</server>
    </updateservers>

--- plugin code

defined('_JEXEC') or die('Restricted access');

use Joomla\CMS\Component\ComponentHelper;
use Joomla\CMS\Factory;
use Joomla\CMS\HTML\HTMLHelper;
use Joomla\CMS\Language\Text;
use Joomla\CMS\Router\Route;
use Joomla\CMS\Plugin\CMSPlugin;

class plgContentplg_zhosmmap extends CMSPlugin
{

protected $app;

public function onContentPrepare($context, &$article, &$params, $page = 0)
{

	$article->text = "Hello";
}

}

My plugin name
filename plugin="plg_zhosmmap"

and plugin class
class plgContentplg_zhosmmap extends CMSPlugin

it is not processed

if I change name and files to any other (for example plgzhosmmap), - it works

Probably I must to name my plugin without plg_, but I did it like the other extensions like rules for j3 (component = com_, module = mod_, and so on)

Just look at how all the core plugins are named and structured.

I think (don't know for sure) it's this line that is the reason that plugin base file/classes don't work any longer if they start with plg_ or (which is a bit harsh) contain plg_
https://github.com/joomla/joomla-cms/blob/4.2-dev/libraries/src/Extension/ExtensionManagerTrait.php#L87

Later in the chain loadPluginFromFilesystem() can't find the plugin base file, better: can't find the class:
https://github.com/joomla/joomla-cms/blob/4.2-dev/libraries/src/Extension/ExtensionManagerTrait.php#L192

That's new but expected from my point of view. Maybe the plugin lagacy mode works different??

Closing as having a pull request. Please test #39954 . Thanks in advance.