To elaborate the last major overhaul on mobile like that I can thing of when iOS transitioned to metal API.
Some of that software was written in engines that had been EoL because of how much marketshare Unity ate up. So those we had to choose to axe or re-make. Some had engine updates but were huge undertakings (drawing a blank on the name of the engine of the one I was actually able to update, took a long time and I had to edit the engine itself to make things work since it wasn't 100%).
Unity as mentioned above had to upgrade unity versions. Anything older than 2017 had to be upgraded to 2017 first and I had to stop there and do massive conversions at that point. Things like the particle system we were using was axed in 2018 and 2017 was the only version that had both so had to do the migration there. They had also overhauled how skeletons and other things worked and basically everything was severely broken porting it to 2017. Only after it was fully fixed in 2017 could I upgrade to anything newer than 2017. 2018+ Also had the new package manager so I had to tear out all the manually imported libraries and import the new libraries via the package manager. Actually had a meeting with Unity staff shortly after spending months converting software and might have torn into them a little bit over that. Some of those projects I was converting were coming out of their source control that they also decided to axe so had to convert all the meta files to git compatible at the same time.
The island camera caused similar nightmares. Suddenly buttons are where the camera is, or it is cutting off text because that was never a concern before those elements made it to the phones, a lot of people had to re-visit and republish their software.
Basically instead of dozens of players there are only a very small number of engines that even survived those transitions in the first place. So this is probably not as severe as it was in the past for a lot of people. But regaurdless all I think of with an announcement like this is having to go back, update and republish everything. May not be today but will happen because that ANGLE system probably won't be around forever just like how the metal transition period wasn't around forever.
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