First, the OSI model is just a model, and things in the real world do not necessarily match. In particular, OSes, do not implement the Session and Presentation layers separately from the Application layer.
The network (Internet for the TCP/IP model) layer is connectionless. The two most-used transport protocols are TCP and UDP. TCP is connection-oriented, but UDP is connectionless.
Since the OSI model is an ISO standard, the ISO/IEC wording is what you should go by.
As you read in the ISO/IEC 7498-1:1994(E), the Transport Layer is defined as both connection-oriented and connectionless. There are explanations about using it both ways (I highlighted the relevant text in this section, but the entire document speaks to connectionless-mode):
220.127.116.11.1 The Transport Layer uniquely identifies each session entity by its transport-address. When providing the
connectionless-mode service, the Transport Layer provides a
connectionless-mode service which maps a request for transmission of a
transport-service-data-unit onto a request to the connectionless-mode
network-service. In connection-mode, the transport-service provides
the means to establish, maintain, and release transport-connections.
Transport-connections provide duplex transmission between a pair of
session-entities (through transport-SAPs).
The ISO definition of the OSI model is the definition of the OSI model. Anybody else who writes anything that disagrees with the ISO definition is making up something that is not the OSI model. Just because it is published in a web page does not make it true. It is true only if it agrees with the ISO definition of the OSI model.
I found a paper that lays out the update discussions to the original OSI model. It appears that in the original model, there was no connectionless mode, even in the Network Layer. In any case, this was all hashed out well before the Internet went commercial in 1995, and any web sites that claim the Transport Layer cannot use connectionless protocols are simply incorrect. The current OSI model, available since before the Internet went commercial, has always included the connectionless mode. It's like reading the reports by doctors from the 1940s and 1950s claiming that smoking is good for you. I know people who still quote those doctor recommendations. Networking moves orders of magnitude faster than medicine, and the original OSI model was out of date before the Internet went commercial; it has been obsolete for 24 years. and anyone who cares about the original version far out of touch with reality.