[NeutralizeR]

Finally, i've found a way by myself...

How to mass select all of the soft deleted posts listed under User CP?

Open your Admin CP > Styles & Templates > Moderation Templates > moderation_posts template:

Find:
$totalposts (the first one)

Add right after $totalposts:

<input type="checkbox" name="allbox" id="checkall_all" title="$vbphrase[check_uncheck_all]" onclick="inlineMod.check_all()" />

By default, only 4 soft deleted posts listed under User CP moderation menu.

How do list X soft deleted posts under User CP moderation menu?

Open your /forum/moderation.php file with your text editor.

Find:

sanitize_pageresults($totalposts, $pagenumber, $perpage, 200, 4);

Change '4' with 'X'... I've changed mine to '100' because of the massive soft deleted posts amount...

Save and upload the edited moderation.php file.

(You may want to revert 100 to 4 after you remove all soft deleted posts)