Is there a good example for acknowledged, connectionless network protocol?

According to what I've read, there are three main connection types in computer networks:

• Unacknowledged, connectionless (e.g. UDP)
• Acknowledged, connectionless (e.g. ?)
• Acknowledged, connection-oriented (e.g. TCP)

From memory those three connection types are defined by the OSI network model, and while the TCP/IP and OSI Models may share some similarities the support of certain features required/implied by OSI does not necessarily translate into support in the TCP/IP model.

Everything I can find regarding "acknowledged, connectionless" protocols simply refers to it being a feature/requirement in the Data Link Layer of the OSI model

While the OSI model is a good model for learning how a network should work, is not necessarily perfect for widespread usage and some of the features are simply not worth the effort to implement in common networking protocols.

The two types you listed (TCP and UDP) are enough to support 99.9% of situations that are likely to occur, and as connections "cost less" with faster networking devices and features like Selective Acknowledgement the requirement for acknowledged yet connectionless protocols becomes minimal, and if you truly don't want a connection (i.e. broadcasting to multiple computers) then acknowledgement could create a processing bottleneck coming back to you.

The only real use I can see for that particular type of connection is in a system like a factory where you absolutely must know that certain data has been received by all machines and I cannot see why, with modern hardware speeds and capabilities, you would want to use a connectionless protocol as a connection-orientated protocol would provide much better quality of service.

TFTP uses UDP, and relies on acknowledgements to function.