Not sure about this patch:
Should not the default be to ON
It looks to me that one never wants to get the MSWord html mess, therefore should not it just be added as always true?

I totally agree with you, I'm just not familiar with the Joomla policy about changing the default behavior. And because in Joomla this feature is not used so far, then to not change habitual behavior, I turned off this option by default.
If you want, I can change it.

I see now better the issue in your patch:
You added the paste plugin in "Advanced mode" although we never include the plugin there

As we indeed only propose the paste plugin in Extended mode, this change

• plugins : \"table link image code charmap autolink lists importcss\",
• plugins : \"table link image code charmap autolink lists importcss $cleanUpPlugin\",

is useless
as well as the definitions for $cleanUpPlugin as the paste plugin will be loaded by

if (!$paste)
+           {
+               $plugins[] = 'paste';
``
+           }

Also please indeed default option to ON.

If I remove 'paste' plugin from "Advanced mode", then 'clean up' option will works only in "Extended mode".
So the question is: should it work in Simple and Advanced mode or only in Extended?
If it should work in Simple mode, then should I move option from 'Advanced' tab to main tab?

Go for Simple and default ON

As we use Paste only in extended mode, I guess we should implement clean up on paste there only.
This would be the most B/C imho

@tested=OK

Ok, now 'clean up' option works only in extended mode. So to test it, you need to switch to extended mode.
I hope in newer versions of Joomla! (at least 4.0) we can turn on this option by default for all modes, since B/C there is not so important.

Incidentally, in the extended mode clean up happens without this PR (perhaps in another way). Therefore, if the use of cleaning up is not planned for the simple and advanced mode, this PR can be closed.

I suggest we discuss this on list

also, we just merged Tiny 4.0.28 version. Please test again with master

It seems that parameter 'paste_auto_cleanup_on_paste' not work in Tiny 4.
If 'paste' plugin is loaded then clean up on paste works, otherwise - not.

Changes necessary to add 'clean up' to extended mode only:
https://github.com/tQuant/joomla-cms/compare/cleanUpExtendedMode
To all modes:
https://github.com/tQuant/joomla-cms/compare/cleanUpAllModes

@tQuant can you update your branche to staging?

I have tested this item unsuccessfully on 9c93fa0

Still stuff added like "" and on...

I am closing this as it can not work
The code in this PR simply called a tinymce version 3 plugin
We are using tinymce 4 now and the plugins for tinymce and their syntax is different