Was it made by RETASPRO in the later period of Japanese comics?

..... How to put it, questioner, your understanding of this software is still a little biased. Retas! PRO is not a post-production software. To be precise, it should be a comprehensive two-dimensional paperless animation software. It includes four modules: scanning, paperless animation, coloring and synthesis. These four modules are all open source and can be used independently. In Japan, AE should still be used in the later stage of animation. Of course, there are also companies that produce and use RETA (such as "Green Six").

Therefore, if we talk about the simple animation later stage, I think there are more mainstream AEs, and of course RETAs has a high usage rate in Japan.

There should be very few post-synthesis with FLASH. The synthesis module in RETAS is CoreRETASHD, you can go and have a look. Of course, you can also learn AE, which is very practical.