How to et stats for Aave onchain liquidations?

You can query historical liquidation events from the protocol subgraphs. There's an example here for an individual user or you query for all liquidation calls with:

{
liquidationCalls{
    id
    user{
      id
    }
    collateralUserReserve{
      reserve{
        symbol
        reserveLiquidationThreshold
      }
    }
    collateralAmount
    principalReserve{
      symbol
      reserveFactor
    }
    principalAmount
    liquidator
    timestamp
  }
}