LunaticBuzz
The lead project maintainer of Obtainium says this:
Obtainium will continue to work on non-certified devices.
https://github.com/ImranR98/Obtainium/discussions/2699#discussioncomment-15418925
So Obtainium will continue to work fine on GrapheneOS.
Its the stock android people who will effectively lose access to Obtainium.
It will likely lead to a reduction in this app's community-based resources (crowd-sourced configs for "hard to add" apps, etc).
But Obtainium will likely stick around despite this.
Even so, alternatives in include:
Gplay
Aurora
F-droid
Accrescent
Or my favorite, install website as PWA via browser instead of using the app