From a National Perspective, Allen's improvement could be seen in the Dallas game on Thanksgiving.
If you hadn't watched him in 2018, and were only looking at numbers, that was the game, to me, that showed real improvement in his ability to stay calm in the pocket, and the Bills figuring out that they needed to shorten his average depth of target and take the middle of the field.
It was that game that he showed he can be accurate by firing lasers in the 10-20 yard range.
Coming out of 2018, the Bills also put emphasis in improving the depth and competition of the offensive line with veteran FA signings. The receivers we have now fit the mold of Allen's strengths. He is a fastball thrower on intermediate routes. The WR's are smaller, quick, and get separation on those types of routes. Top three are all accomplished veterans.
Also, he continues to work in the offseason with Jordan Palmer, who has outlined the improvement in Allen's throwing motion.