My experience growing up was that my Dad would program 'EEPROMs' or Flash ROMs from his Apple IIGS. (I don't know if that is similar to an FPGA or not). He used these in custom wire-wrap computers he was building (around 1986-1989).
We know that the first FPGAs went on sale in 1984.
I've just finished reading Charles Petzold's book, Code: The Hidden Language of Computer Hardware and Software. In it Charles explains building relays into gates, gates into logic components, and logic components into computing machines.
He talks about the book TTL Data Book for Design Engineers which first came out in 1973. (I'm assuming people had lists of TTL gate chips prior to this). This talks a lot about the Texas Instruments 7400 series of logic gate chips.
Now to me it seems you could combine TTL chips (or relays or transistors) into a CPU using the instructions in this book. But everything online says "use FPGAs instead! It's easier for that scale! [Unless you're getting the concepts and want to see the end to end view]". Fair enough - some pragmatism is good - it depends on what the outcome is.
So somewhere between 1948 and 1984 someone must have thought of the idea of FPGAs.
Assumption: By "people" I mean any hobbyist or engineer who is not explicitly prototyping the next FPGA to be manufactured by Naval Surface Warfare Center, Altera and Xilinx.
My question is: Were people building FPGAs out of TTL logic prior to the first sales in 1984?