theme engine is new in LS3
Please, read this:
https://manual.limesurvey.org/New_Template_System_in_LS3.x

Yes ? And :
In 2.6lts (and surely 2.5X) : you can hack completed.pstpl to show {SAVEDID}
In 3.X : you can't , even using new system {{ processString('{SAVEDID}') }}

There are an issue here …
About SAVEDID : can not fix before 4.0 (need EM update), but it must be fixed …

sorry, didn't see you was the reporter, I thought about usual complains about retro-compatibility.

It's about retro compat : but not without updating theme/template ;).

I try with
https://bin.shnoulle.net/?eb9cb001c0d38e9e#EMdYT04bSvSFod58mpxxEpPihNIYCNgzt2WwORnWqjU=
(partial)

But need to move Yii::app()->twigRenderer->renderHtmlPage to public.

I can make the pull-request

This fix partially : OK with EM Q00.NAOK etc … but not with SAVEDID because it was in replacements_helper and not in EM.

I think SAVEDID can be moved to EM (like SID and TOKEN:} in 4.0 but not in 3.X

https://github.com/LimeSurvey/LimeSurvey/pull/1206

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=28782

Same for TOKEN then SAVEDID : https://www.limesurvey.org/forum/can-i-do-this-with-limesurvey/117616-get-token-in-completed-submit-twig-template#180175

I close this one for end message
I open another one for SAVEDID
https://bugs.limesurvey.org/view.php?id=14507