
Samsung Galaxy S5
The latest firmware for Samsung Galaxy S5 (SM-G900L) is G900LKLU1CPL5 with Android 6.0.1 (Marshmallow), released on December 15, 2016 for 1 region (CSC). Every official firmware build for SM-G900L, newest first: one row per build with all regions (CSC) that received it.
SM-G900L started on Android 4 and has received 2 major Android upgrades; it now runs Android 6.0.1 (Marshmallow). The newest build, G900LKLU1CPL5, was released on December 15, 2016. No new build has appeared for 120 months, so regular updates for this model have most likely ended.
Specifications of SM-G900, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-G935TZK

SM-G930AZK

SM-G935AZS

SM-G935TZS

SM-G935AZK

SM-G935TZD

SM-G975WC

SM-G975FC
The newest firmware for SM-G900L runs Android 6.0.1 (Marshmallow): build G900LKLU1CPL5, released on December 15, 2016.
The latest build for SM-G900L, G900LKLU1CPL5, was released on December 15, 2016 (9 years ago).
SM-G900L has had no new firmware for 120 months, so regular updates have most likely ended.
Yes, all firmware files for SM-G900L are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G900L, 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 SM-G900L.