Let’s start with data from previous expansions that can hint at when release is based on duration of pre-release events.
The Burning Crusade
Alpha + Beta: 150 days
Beta: 96 days
Prepatch: 42 days
Wrath of the Lich King
Alpha + Beta: 218 days
Beta: 119 days
Prepatch: 30 days
Cataclysm
Alpha + Beta: 218 days
Beta: 160 days
Prepatch: 56 days
Mists of Pandaria
Alpha + Beta: 188 days
Beta: 188 days (no alpha)
Prepatch: 28 days
Warlords of Draenor
Alpha + Beta: 162 days
Beta: 139 days
Prepatch: 30 days
Release before deadline: 37 days
Legion
Alpha + Beta: 281 days
Beta: ?
Prepatch: 42 days
Release before deadline: 22 days
Battle for Azeroth
Alpha + Beta: 188 days
Beta: 112 days
Prepatch: 29 days
Release before deadline: 38 days
Average
Alpha + Beta: 200 days
Beta: 136 days
Prepatch: 37 days
Release before deadline: 32 days
Shadowlands
Alpha started: April 4th
Beta started: July 15th
Deadline: December 31st
Since some expansions have only duration of combined Alpha + Beta, that is more reliable, so let’s use that. Based on 200 days of Alpha + Beta, Shadowlands would release October 26th.
We can also take the average days released before deadline, as they would set deadline based on when they expect release, plus some margin. Using this Shadowlands would release November 28th.
So, based on just averages, we have a probable release date span of October 26th - November 28th.
Another piece of data is that there was a new broom model added, likely used for Hallow’s End event starting on October 18th. If we assume this will be part of pre-patch, we can expect pre-patch to hit latest on the Wednesday before, October 14th.
Another tidbit is that the first week with no new corruption resistance to gain will start September 16th, thus it is unlikely pre-patch would hit before that.
Pre-patch September 16th - October 14th maybe?
Then if we use this along with the usual pre-patch duration, release would be October 22nd - November 19th.
This narrows our release date span down to October 26th - November 19th.
However, I am going to guess we will have some time to play with maximum corruption before pre-patch, about a month or so. And we also know the releases will be on a Wednesday in EU.
Thus, my guesses are:
Pre-patch: October 7th
Release: November 11th