OP is mistaken.
Warp Charge damage bonus and Brain Burst damage were buffed in February 2023:
Warp Unleashed was buffed (to also affect BB damage) in March 2023:
There have not been any buffs, nerfs or other changes regarding WC, BB or the WU feat since then.
Right now, BB damage is the same as it was right after the March update.
Since the March update, BB can 2 shot Crushers on damnation.
This requires both BB to be cast at relatively high peril (for strong effect from Warp Unleashed), while the average number of active WC for the two BB casts must be ⋝ 5 (5+5, 4+6, 5+6 or 6+6).
The buff to Warp Unleashed also made it possible to 1 shot damnation Flamers with BB.
One option to 1 shot Flamers is to start casting BB at 0% peril while having 6 WC (peril gained from charging the BB grants just enough damage bonus from Warp Unleashed).
This only works at 6 WC, but not at 5 WC (+4% dmg per WC).
These specific breakpoints have existed since the March update, and they still work as of now (just tested them).
They would no longer work, if the 10% buff from the February update was reverted (this is especially obvious and easy to reproduce for the one against Flamers, where you can start casting at exactly 0% Peril and which only works with 6 WC).
So the BB damage was not reduced back to pre buff values (at least not the damage against carapace and maniacs).