Official patch notes from sources like Serebii and Nintendo Support highlight the following:
The update data for Pokémon X is different from Pokémon Y ; you must ensure the update matches your specific game version and region (EUR or USA). Pokemon Y: Update 1.5 [Decrypted] 3DS (EUR/USA)...
is the final official software update released for the Nintendo 3DS title. While the eShop has largely closed, game updates like this remain available to download for users who own the software. Update Overview Release Date: April 22, 2015. Official patch notes from sources like Serebii and
Like previous versions (1.1–1.4), version 1.5 includes cumulative fixes for older issues, such as the Lumiose City save glitch and GTS communication errors. Installation for Decrypted/Emulated Systems Update Overview Release Date: April 22, 2015
Are you encountering a (like a 002-0119 or 005-4222 error)? How to Update Pokémon X and Pokémon Y | Nintendo Support
The update primarily focused on "various bug fixes" to improve the overall gaming experience.
Fixed a specific issue that caused players to disconnect during Random Matchups and Online Competitions.