Samsung Galaxy S5
The latest firmware for Samsung Galaxy S5 (SM-S903VL) is S903VLUDU6AQF5, released on August 2, 2017 for 1 region (CSC). Every official firmware build for SM-S903VL, 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/S903VLUDU6AQF5
The newest build, S903VLUDU6AQF5, was released on August 2, 2017. No new build has appeared for 112 months, so regular updates for this model have most likely ended.

SM-S906L

SM-S9080

SM-S9010

SM-S902L

SM-S918Q
The latest build for SM-S903VL, S903VLUDU6AQF5, was released on August 2, 2017 (9 years ago).
SM-S903VL has had no new firmware for 112 months, so regular updates have most likely ended.
Yes, all firmware files for SM-S903VL are 100% official, original, and obtained directly from Samsung servers.
To flash SM-S903VL, 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-S903VL.