PS5 cross-gen support will be mandatory for all newly submitted PS4 games from July 13.
As Eurogamer reports, updated internal documentation sent to PlayStation partners now asks developers to confirm that their game - that is, the submitted version of their software - is also compatible with PS5 hardware. All PS4 games submitted after July 13 must be fully compatible with PS5, offering the same features and functionality as on PS4. Games submitted before that date won't require PS5 support, but it's apparently "strongly recommended" by Sony.
Note that this rule applies to games that are submitted for PS4 release, not games that are released on PS4. Sony will determine the necessity of PS5 support based on a game's submission date, not its release date. In other words, PS4 games that are released after July 13, like Ghost of Tsushima, will not necessarily be required to run on PS5, as they were undoubtedly submitted well ahead of time.
Sony will reportedly contact developers on an individual basis in order to test and/or troubleshoot PS5 forwards compatibility for new PS4 games, just as it will rely on developers to ensure PS5 backwards compatibility for many older titles. As senior vice president Hideaki Nishino said in March, Sony is "currently evaluating games on a title-by-title basis to spot any issues that need adjustment from the original software developers."
The company has said that PS5 backwards compatibility will include "an overwhelming majority of the 4,000+ PS4 games" already released, and with these new submission requirements, all new PS4 games will eventually be cross-gen as well. However, it remains to be seen how backwards compatibility is actually delivered, and how it stacks up to the multiple generations of Xbox Series X backwards compatibility empowered by Microsoft's Smart Delivery program.
New reports indicate that PS5 backwards compatibility may be tied to rolling catalog updates.