The subsidy is the part of a miner's reward that did not exist before the block. Bitcoin Core computes it from the height alone: 50 BTC, halved once for every 210,000 blocks already mined, in whole satoshis. Every node applies the same function, so a block whose coinbase pays out more than the subsidy plus the block's fees is invalid everywhere.
Because it depends only on height, the subsidy of any block, past or future, can be worked out without asking anyone. It fell to 3.125 BTC at block 840,000 and falls to 1.5625 BTC at block 1,050,000.
The Mining tab shows how much of the last 144 blocks' reward came from the subsidy and how much from fees.