There was no aftermath. This process is still happening into the present day. The Age of Apostasy was a good choice for introducing new technology because after the Horus Heresy, the Imperium was due for a period of quiet and expansion. Instead, there was a power struggle which, given the size and complexity of the Imperium could have science and technology projects starting and stopping, even being lost for centuries while the government was restructured again and again. The hole left by the Emperor caused a power struggles between the Ecclesiarchy (the voice of the Emperor) and the Administratum (the bureaucracy of the Imperium)
There were new sciences being redeveloped during this period and it is easy to see technology developed during that time being challenged and shelved as the two groups struggle for dominance for the next 7,000 years.**
This is a good time period for Games Workshop to develop "new" technology because it could be part of the expansion of the Imperium and filling technological holes discovered by the company, writers, or gamers. Retcons like these are part and parcel of GW development.
Given the fractious nature of the Warhammer 40K universe, the uneven pace of technology development often due to the superstitions surrounding technology by the techpriests, and adding the actual erosion of technological knowledge) some technologies may be used in one section of the galaxy and not in others.
The Ecclesiarchy is the ruler of the Imperial Cult, the bearer of the sacred word of the Emperor. It spread into the lives of every citizen and servant of the Imperium, from the most humble miner and scribe, to the greatest heroes and the most influential officials, including the High Lords of Terra. The High Lords believing, like every other citizen, that the Ecclesiarchy was speaking for the Emperor, would often follow their words in directing the Imperium. Though they did not actually speak the words of the Emperor, the Ecclesiarchy did little to retract this image, and soon they were indirectly controlling the fate of the Imperium, influencing legal changes, military deployments and organization, threat priority decision-making and where Imperial resources should be directed, often towards themselves.
Eventually, the power of the Ecclesiarchy grew too much and the Fabricator-General of the Adeptus Mechanicus, along with a group of Space Marine Chapter Masters rallied together, and eventually the Administratum joined, to fight against the strength, will and control of the Ecclesiarchy.
The Administratum, often thought of as the centre of the Imperium, was angered at its loss of power. This began a feud that lasted for at least 7,000 years, has continued to the present day, and is likely to continue far into the future. The Administratum began to use its position to undermine the power of the Ecclesiarchy. They could influence voting and position its own loyal followers in other departments. From M34 to M35, the power of the Ecclesiarchy waned and, following the election of a series of weak Ecclesiarchs, the Administratum managed to wrestle back control of the Imperium. They continued to gain power at the expense of the Ecclesiarchy, although allowing them to keep their external image as dictating the words of the Emperor and ruling the Imperium, while the Administratum actually had the power.
As with most things Warhammer 40K and Games Workshop delivered, when a new model is created or envisioned, it is usually to deal with player complaints about how real militaries move and how the game envisions the Adeptus Astartes SHOULD move given their technology and the march of time.
Oversights including limited air-power, armored personnel carriers, grenade launcher effectiveness, orbital bombardments, orbital insertions have all seen strange additions to the game and yet the fiction survives. New writers will work them into obscure battles and if they come to be loved, then you may see them more in the future.
Such new additions allows Games Workshop to sell new versions of the books, games and support materials in addition to newer figures and model designs. They have to keep new product moving.