Destiny 2 may be on Steam now, but if you've not added another account to your Bungie profile since playing on Battle.net, you'll need to hop to it.
That's because Bungie has confirmed that "Blizzard's Battle.net accounts will no longer be available for users to log in with on Bungie.net or the Destiny Companion app, nor will users be able to create Bungie profiles using Battle.net credentials."
Destiny 2 Players Must Migrate Battle.net Accounts Today Or Lose Their Profile Forever
That's according to a Bungie support page, but it does note that "Users who have other platform accounts linked to their Bungie profile will still be able to access their profile."
If you haven't moved your account to Steam yet, you risk losing your character and progress if you don't move before 9 AM PST.
How To Link Accounts
As per Bungie's account linking page:
If a player engages with Destiny 2 on multiple platforms, they may wish to link multiple authentication methods to a single Bungie.net profile. To do this, follow these steps:
"Please Note: Only one of each type of platform account may be added to a Bungie.net profile. A Bungie.net profile may have up to one PSN ID, one Xbox Gamertag, one Stadia ID, AND one Steam Account linked, but cannot have multiples of any. Furthermore, PlayStation Network accounts may require more details, such as the creation of a PSN ID, before the account may be linked."
Even if you're not planning to return anytime soon, it's worth linking whatever platform you can just so that you have the option to do so in your own time.