TTL is just an implementation variant of DTL, which had some advantages for low supply voltages.
When making DTL circuits with bipolar transistors, especially at low supply voltages, in order to make the logic levels at the output equal to the logic levels at the input, a simple solution was to add a diode at the output of an AND gate made with diodes, to shift down the logic levels by the voltage drop over that diode.
When the diodes used for gates were bipolar diodes, not Schottky diodes, they stored a big electric charge when on and they could be turned off quickly only if there was a path to evacuate the stored charge. Also the bipolar transistor of the inverter stored a big charge in the base, which had to be evacuated quickly for turning it off.
The current needed to evacuate the stored charge was required to pass in reverse direction through the level-shifting diode, which is not possible, so the DTL gates with level-shifting diodes were slow.
In bipolar IC technology, all diodes are made from transistor junctions, in order to not have separate process steps for making diodes. The junctions suitable for fast diodes are the emitter-base junctions. So all the diodes of a DTL gate were made inside an IC as multiple emitters of a transistor, with a short over the base-collector junction, to disable the transistor effect and make it work as a bunch of diodes.
At this point in the history of integrated DTL circuits, someone made the observation that seems trivial in hindsight, that removing the short over the base-collector junction allows to use it as the level-shifting diode, saving a diode.
Moreover, this not only saved a diode, but the bipolar transistor effect results in passing current through the reverse-biased level-shifting diode, allowing the fast turn off of the inverting transistor.
So in those early times, when DTL circuits could be made only with bipolar diodes and with bipolar transistors, TTL was the best variant.
Later, when the bipolar diodes were replaced with Schottky diodes, which store negligible charge when on, and when diode clamps were used over the inverting transistor, so that it no longer reached deep saturation and it no longer stored a big charge, TTL was no longer the optimal implementation and some of the so-called Schottky TTL families were actually DTL circuits, not TTL, but they had retained TTL as a marketing term, as this had become almost synonymous with bipolar logic integrated circuit.
TTL could never be used in discrete circuits, because it is based on bipolar transistors with multiple emitters and/or multiple bases, which have never been available as discrete parts.
diodes are cheaper, smaller, and easier to solder than transistors; ttl took off with integrated circuits
ttl was the default choice for building discrete logic circuits until about 01980, after which point it was obsolete because cmos (74hcxxx, not cd4xxx) was better in every way except esd
if you're running an educational computer lab on a tight budget, esd is still the dominant consideration, because students will burn out all your cmos chips with static after only a few dozen uses, while ttl chips will survive most of their mistakes
but outside a circuit lab or possibly repair of 40-year-old devices there's no reason to use ttl
It depends how "discrete" you mean. If you're using gate ICs, TTL has most of the advantages. If, for some reason, you're limiting yourself to discrete transistors, TTL is going to be hard to implement. These transistor-based projects are mostly done for the bragging rights.
When making DTL circuits with bipolar transistors, especially at low supply voltages, in order to make the logic levels at the output equal to the logic levels at the input, a simple solution was to add a diode at the output of an AND gate made with diodes, to shift down the logic levels by the voltage drop over that diode.
When the diodes used for gates were bipolar diodes, not Schottky diodes, they stored a big electric charge when on and they could be turned off quickly only if there was a path to evacuate the stored charge. Also the bipolar transistor of the inverter stored a big charge in the base, which had to be evacuated quickly for turning it off.
The current needed to evacuate the stored charge was required to pass in reverse direction through the level-shifting diode, which is not possible, so the DTL gates with level-shifting diodes were slow.
In bipolar IC technology, all diodes are made from transistor junctions, in order to not have separate process steps for making diodes. The junctions suitable for fast diodes are the emitter-base junctions. So all the diodes of a DTL gate were made inside an IC as multiple emitters of a transistor, with a short over the base-collector junction, to disable the transistor effect and make it work as a bunch of diodes.
At this point in the history of integrated DTL circuits, someone made the observation that seems trivial in hindsight, that removing the short over the base-collector junction allows to use it as the level-shifting diode, saving a diode.
Moreover, this not only saved a diode, but the bipolar transistor effect results in passing current through the reverse-biased level-shifting diode, allowing the fast turn off of the inverting transistor.
So in those early times, when DTL circuits could be made only with bipolar diodes and with bipolar transistors, TTL was the best variant.
Later, when the bipolar diodes were replaced with Schottky diodes, which store negligible charge when on, and when diode clamps were used over the inverting transistor, so that it no longer reached deep saturation and it no longer stored a big charge, TTL was no longer the optimal implementation and some of the so-called Schottky TTL families were actually DTL circuits, not TTL, but they had retained TTL as a marketing term, as this had become almost synonymous with bipolar logic integrated circuit.
TTL could never be used in discrete circuits, because it is based on bipolar transistors with multiple emitters and/or multiple bases, which have never been available as discrete parts.