They don’t.
Therefore no need to opt out.
Securities Lending is a perfectly legitimate operation but it HAS to be explicitly disclosed to it’s clients (you) via the T&Cs.
But for the record I’d like to see FT lend stock. It’s virtually zero risk, it moderate/medium operationally problematic and it generates an income that can (should) be shared with clients (you). You - as a client - are entirely covered by daily mark to market margin requirements and collateral in-lieu of assets lent in the unlikely event of default by the counterparty (which is usually intermediated anyway - effectively an insurance policy)