Bungie released another weekly blog post, talking about the imminent monumental changes Destiny 2 will undergo. Players will be able to transfer their accounts from 20 August 2019 and the game will be released on Steam on 1 October 2019.
Destiny 2 migration to Steam will be completed on 1 October 2019 but players will have over a month to transfer their accounts from Battle.net. Following the migration, Destiny 2 will not be available on Blizzard.net so the move will be mandatory for the existing players.
The good news is that everything, including guardians, gear and triumphs will be carried over and players will essentially just drop one launcher for another that happens to have more features and seems to be more stable. All the existing players will be invited to start the account linking process on 20 August 2019 and they can complete it before the Steam release in order to make the transition as smooth as possible.
Cross saving will come one day later, on 21 August 2019. It is a great feature that will allow players to copy their progress from one platform to any other supported system but it has some drawbacks.
For example, nothing that players purchased will transfer to the other platform. This includes expansions and Silver balances but Bungie didn't specify if purchased cosmetics will carry over.
The reason for the inability of carrying purchases over is more likely that the different platforms don't allow using items paid for in another store rather than Bungie not wanting to carry them over.
It does benefit the developers indirectly since players will have to purchase them all over again if they want their shiny cosmetics from consoles to be displayed on PC but it is really hard to imagine Sony letting microtransactions from Xbox One carry over to PlayStation 4 and vice versa.
In case that you are looking for more information on cross saving, you can check the official FAQ or perhaps even contact Bungie support for information not included in the list.