TTL = transistor transistor logic

This is the technology in the common 74' series integrated circuits that became popular in the late 1960's and 1970's. You can get basic IC's that perform AND, NOT, XOR operations but also more complex IC's that perform as multiplexors, buffers, registers and so forth. You can find these in computers like the PDP or VAX series machines.

They are not used as much today and so less of them are still manufactured.

Many, many of my favorite 74' parts from the old days are no longer made. The MSI parts that were bread-and-butter of this kind of design back then make less-than-zero economic sense any more. Nobody would design them in, so nobody makes them.

Today you can get the basic logic, of course, because a little bit of glue and duct tape is still necessary. And the super basic shift registers and I/O buffers are still useful. But all computation has moved into much more highly integrated chips or FPGAs.

Fewer chips.