Should be early December.
The Linux version was not 'pushed back'.
The console versions have an extra certification process that they needed to go through, as well as manufacturing for the retail release. The PC versions of the game are digital only, and therefor had a more flexible release, which did not need to be determined as far in advance (and it was only fairly recently that Larian was confident the Windows version would be ready by the console release).
Without separate teams for every platform, the only way the game could be completed was to do the content first and focus on the console versions. When they got sent for certification, start switching over to the Windows version to focus on PC only issues (changes to the keyboard/mouse UI, etc), then port to Mac and Linux.
The plan was to have that process done for a simultaneous release, but it took longer than anticipated.
Without the console version to justify the additional development, D:OS wouldn't be getting the Enhanced Edition. It could have gotten another update or two, but not 7 months of voice work, changes to story, a rewritten ending and additions/changes to dialogue and quests throughout the game, etc.
