How do I turn off the end of support banner in office 2010 permanently? (Office 2010, Windows)

I'm not sure I understood the 1st part. But as to seeing the whole key missing, would it help if most of the privacy options are unchecked in Office? As in the following:

I tried unchecking all of the "options" under Trust Center a month or more ago and none of that worked so I am very glad I found this thread.  As an aside the color of the Banner recently changed to a dark pink or light red; it was a lighter color before, as I recall yellow.  I suspect that when support stops it will be even more obnoxious like bright red.  I wonder which key edits the color, not that I will change it.  Also I can confirm from my end that 0:en is for Word, 1:en is for Excel and 3:en is for PowerPoint.  I don't have a 2:en.

I don't have a 2:en either. That probably is for OneNote. The two fixes I mentioned (changing TimeToNextcall in the Header to year 2025 and MessageScenario in the Body to zero) seem to work as I haven't had that banner pop up. After Oct. 13 they may come up with a new twist. We'll cross that bridge later. The reason I turned privacy options off as well as updates is because I don't want Microsoft to detect that I have Office 2010. But as 007 mentioned, they may continue updates even after expiry, so it wouldn't hurt to periodically check for updates.

I found the TargetedMessagingService on both HK_CU and HKEY_USERS.  I did the edits on HKEY_USERS and it also edits the entries on HKCU automatically.  Can you tell me the difference between HKCU and HKU?  Why do I have both?  I am not an expert on RegEdit (to say the least).  TIA

I edited the entries (on HKU) as you suggested, 2025 and 0, for both Word and Excel.  I did not edit PP as I rarely use it.  FWIW the entry under 3:en (for PP) originally showed a 2017 date, likely the last time I used PP, so it fetched that from somewhere in the registry.  I then opened PP, got the banner, and the "Time to Next Call" changed to 2020-9-21 (I did this yesterday, 9-18) so it reset the next call to 3 days from the last time I opened PP.  When Microsoft first started the obnoxious banner it appeared every few weeks as I recall and now it is every 3 days.  Would not surprise me if after October 13 it is everyday.

I wonder where in the registry they set the 3 days?  Under "Header" there is an entry named "CallDelta" but I cannot tell what the current data entry means.  Can you or others (007) tell me?  Is that how they set the days?

Edit: Sorry, more confusion.  I edited the Next Call on Header for 03:en (i.e. PP) to yesterday, 2020-9-18T03:38:??Z and then opened PP again.  Got the banner as expected but now the registry shows the Next Call at 2020-9-21T18:07:36Z so it did not reset by 3 days, just a little over 2 days (it is 9/19 13:13 here) so I wonder where and how they reset the Next Call.

You are right, those keys appear in HKEY_USERS too. However, I only changed HKCU and the HKEY_USERS changed automatically.  HKCU is just a link, the same info is available in both. As to your other questions, perhaps someone else can answer as I'm no expert either.

Change MessageScenario, within the Body, from value 6 to value 0 message goes away.