If not, suppose I send out a transaction that's time-locked to 1 day/month/year/decade into the future. To what degree of certainty will my transaction be eventually be included in a block?
Your transaction is basically guaranteed to never be included in a block if all you did was broadcast it before the locktime was reached.Nodes do not accept or relay transactions which cannot be included in the next block. If you broadcast a transaction with a future locktime, basically all nodes would drop it. Besides that, nodes will also evict transactions that have been unconfirmed for too long (typically 2 weeks as that is the default in Bitcoin Core). If you had a transaction with a timelock greater than 2 weeks in the future, and it was somehow accepted into some node's mempool, it would be dropped before the timelock was reached.
