We have tried to keep track of Android updates for tons and tons of phone models for the past two years or so. Task is quite massive, but with biggest manufacturers, we have already a pretty valid data in place - and hope to get it even better.
For smaller manufacturers (Tecno, ZTE, etc), confirmed data is almost impossible to gather, so apologies for that (and if you can help us out with trustworthy sources for those manufacturers, let me know).
So.. we thought, how would all that data look like if we'd build a timeline out of it. This is how it turned out: