The idea of deprecation and death of code assumes that those using it can easily stop.
That’s not always the case.
When Flash dies Jan 12, 2021, you’ll see what I mean.
There’s been so much misinformation about it, but there’s an OS datetime check in the Flash viewer code, browsers will disable the plugin even for some older versions not just new releases and have or will remove PPAPI and NPAPI support, and Windows already has an optional update to kill Flash support that will be part of regular updates in Summer 2021.
Even with three years of warning, it’s not ok.
It’s an extreme case of deprecation, being attempted by the best in the business, but it is inherently bad, and there’s hardly a good way of doing it.