Do we get the new SW updates from 945?

You're probably right. But that's no indication that these features are coming to the production firmware for Fenix 5 Plus. What's more likely is that this was just a developers Fenix 5 Plus running a non-release firmware for testing purposes. As far as I'm aware Garmin have never back-ported Firstbeat features to previously released watches because of licensing. But there's a first time for everything I guess? I mean technically they've already done it because they've back-ported these features to the MARQ but that's a ridiculously expensive and more recent watch than the Fenix 5 Plus is.

Apparently they are going to back-port features to the Edge 1030 (https://www.dcrainmaker.com/2019/04/garmin-edge-530-cycling-gps-in-depth-review.html) 

It would make sense if they back-ported / updated to the F5+ the features it already has, VO2 max updated version taking heat and altitude into account and race prediction times and training status. 

Let's hope... unless we can have an official answer to that question. 

Not sure, just wondering - but don't these new metrics (updated VO2max and altitude acclimatation) require the new blood oxygen sensor featured on the x45 range, and MARQ athlete ? 
Would love to see these metrics back ported to my 5S plus if that's possible of course :)

Only altitude acclimation requires the SpO2 sensor to function. The heat acclimation is based on getting a temperature reading of 71f/21c from the last 3 hours based on the weather widget when paired with your phone.

However Fenix 5X Plus already has Pulse OX, so it would be capable of every new metrics...

Since the newer firmware also has all this animation stuff, we probably have to wait for ConnectIQ 3.1 first. There's a demo firmware here but according to some developers, that doesn't contain new features yet.

Actually, the CIQ stuff is different than what you're thinking - it's only for CIQ apps.  I've seen the CIQ animation running on a watch with the 20.01 FW.  That firmware is actually the based 6.00 FW but with a 3.1.0 CIQ VM vs a 3.0.x.  It's got an odd version number so that it doesn't get replaced when new public FW is released (that is until the CIQ developer wants to).  I would not recommend installing 20.01 unless you are a CIQ dev that wants to test some of the new CIQ 3.1.x features, as nothing in the app store will have those features at this point, as the 3.1.0-betas SDK don't allow packaging for the app store.

...aaaand the picture is gone.

Wow. You're right! :-)
Not that this promises anything, Garmin is an American company after all, I guess they'd like to avoid being sued. ^^ Still keeping the fingers crossed.