Can your pruned Bitcoin node serve an old block?
A block explorer needs transaction IDs from height 800,000. Your Bitcoin Core node knows the hash at that height. A second RPC call tells you whether the block data is still on disk.
Find the retention boundary
bitcoin-cli getblockchaininfo | jq '{chain, blocks, pruned, pruneheight}'
Run this on a mainnet node with jq installed. Check that blocks has reached your target. In Bitcoin Core 30, pruneheight is one above the last pruned height. A value of 850,000 puts the example height below that boundary. A node with pruning disabled omits the field.
Read the block your application needs
hash=$(bitcoin-cli getblockhash 800000)
bitcoin-cli getblockheader "$hash" | jq '{height, hash}'
bitcoin-cli getblock "$hash" 1 | jq '{height, nTx, first_tx: .tx[0]}'
The header query identifies the block. The last query reads its body and prints the first transaction ID. If Bitcoin Core reports that the block was pruned, repeat that last query against a node retaining history for this height. When monitoring access to historical data, record the requested height and the outcome of the body read. A successful check at the chain tip only tests recent data.
bitcoincore.org getblockchaininfo · bitcoincore.org getblock