I learned that data storage on NEAR protocol requires to stake NEAR tokens. Is there a way to monitor the total amount of data stored on chain? Didn't find it on NEAR Explorer (or perhaps didn't look at the right place)
The increase/decrease in storage use automatically locks some of the tokens on the account. Explorer displays "STORAGE USED" on the account details page (example). mainnet is configured (see EXPERIMENTAL_protocol_config JSON RPC method for other config options) to have "storage_amount_per_byte": "100000000000000000000", which means that 100kb of data (smart contract code, smart contract state, account access keys, and profile data [liquid balance, locked balance, account id]) locks 1 NEAR on your account (so you just cannot transfer/spend those tokens unless you remove something from the storage).