If you already have the game downloaded on your smartphone then you’ll still be able to play it as long as you don’t uninstall it. However, once July 15th comes and goes the in-app purchases (including “Additional music pack”, “Support characters” and “Additional characters”) will no longer be available either.
The Rhythm of Fighters had you conducting songs from classic SNK games while The King of Fighters characters battled in the background. It was first released as a paid app but changed to a free-to-play game that gave you a few songs for free and more to download as in-app purchases.
You can read more about SNK’s decision to change the game’s payment system in Siliconera’s interview with the company.