Hi Sébastien,
Looking at the 4.27 LTS, it does indeed appear this solution is in line with how the system has evolved equivalently with USE_OS_SMALL_BLOCK_GRAB_MEMORY_FROM_OS being disabled. Removal USE_OS_SMALL_BLOCK_ALLOC altogether would incur a performance penalty. You can consider back porting MallocBinned from 4.27+ should you need to reenable it.
Best regards.