
Samsung GALAXY S4
The latest firmware for Samsung GALAXY S4 (GT-I9515L) is I9515LUBU1BQF1 with Android 5.0.1 (KitKat), released on June 7, 2017 for 1 region (CSC). Every official firmware build for GT-I9515L, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/I9515LUBU1BQF1
GT-I9515L started on Android 4 and has received 1 major Android upgrade; it now runs Android 5.0.1 (KitKat). The newest build, I9515LUBU1BQF1, was released on June 7, 2017. No new build has appeared for 114 months, so regular updates for this model have most likely ended.

GT-I9308

GT-I9268

GT-I9228

GT-I9210

GT-I9108
The newest firmware for GT-I9515L runs Android 5.0.1 (KitKat): build I9515LUBU1BQF1, released on June 7, 2017.
The latest build for GT-I9515L, I9515LUBU1BQF1, was released on June 7, 2017 (9 years ago).
GT-I9515L has had no new firmware for 114 months, so regular updates have most likely ended.
Yes, all firmware files for GT-I9515L are 100% official, original, and obtained directly from Samsung servers.
To flash GT-I9515L, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing GT-I9515L.