It would seem Adobe will kill Flash, but not release its source code under a free license.
So, when will Macromedia Flash’s copyright expire?
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Adobe Inc is a Delaware incorporated corporation, so we should look at United States copyright law. We can assume that all source code written for Macromedia Flash were works for hire and thus the author to be considered Adobe itself.
Title 17 of the United States Code § 302 (c) states:
In the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication, or a term of 120 years from the year of its creation, whichever expires first.
Supposing Adobe Inc final release of Flash was on 2020, its source code would enter Public Domain on 1st January 2116. (2020 + 95 = 2115, plus the remaining of that year)