Few things

1. Yup we haven't tried to get the upgrades working yet so it will almost certainly fail :) (if nothing else because we aren't removing all the old files we're deleting yet)
2. What happens to the Database Fixer tool with the SQL files gone? I mean for sure it's not an upgrade path we support but unsure if the fixer tool is usable without it? It would at least mean people would be forced to run the fixer before they upgrade from 3.9 to 4
3. Everything in script.php looks good :)

What happens to the Database Fixer tool with the SQL files gone? I mean for sure it's not an upgrade path we support but unsure if the fixer tool is usable without it? It would at least mean people would be forced to run the fixer before they upgrade from 3.9 to 4

It would only apply SQL files from 4.0 onwards, and yes it would mean people have their site up to date prior to the upgrade.
Do you think we can't assume people have a working installation before they upgrade?
Maybe we can add a check to the Joomla Update component which shows a warning if the fixer detects an issue to prevent people accidentally upgrading a broken installation?

Imho it's a step we have to take eventually anyway. We have files going back to 2.5.0 in 2011 there...

Is there a reason we do not support updates from 3.8.last to 4.0? As 3.9 and 4.0 are currently planed to be released at the same time?

From a practical perspective, it probably won't be much different unless we remove something from the deleted files/folders list that gets removed between 3.8 and 3.9 and inherently wouldn't be in 4.0. As a matter of policy though, let's keep the main focus on 3.last to 4.0 then take into consideration supporting additional releases as practical.

Can you fix conflicts and I'll get this merged :)

Rebased and conflicts solved.

@wilsonge reminder to merge this

Thanks for the reminder :)

Thanks @Bakual :)