I had looked into TCP/IP stacks several times in the past. To my knowledge there has never been one that supported Workbench 1.3 - This is partly what took me so long to figure this stuff out. Everyone else had been pointing to things like that in order to get "online"... Well, their version of online was via the World Wide Web, they were all using Workbench 2 or above, and most used TCP/IP stacks. I mean I recall having a little fun in the early 2000's using IBrowse via WinUAE in a 3.1 setup, but fun with http on an Amiga today is not what I would call worth my time. Of course this could be achieved on machines such as the 500 or 2000, but Workbench 2 or above would be needed.
I had, and continue to have more fun with the BBS'ing, which has plenty of terminals that work under 1.3, and gives you an authentic 80's/90's experience. For that, something that emulates a modem would surely be best. Personally I'm still interested in the stacks for the Amiga in my 2.04 partition, as I know there are some ICQ type programs that support that there.