I imagine there are servers that drop QoS 1 or 2 messages that never complete the protocol after X amount of time, so that they don't keep consuming resources i

I imagine there are servers that drop QoS 1 or 2 messages that never complete the protocol after X amount of time, so that they don't keep consuming resources indefinitely. MQTT 5 standardizes this by adding a message expiry interval field, though even there it's opt-in by the publisher and defaults to forever.