Samsung Galaxy S5 Active
The latest firmware for Samsung Galaxy S5 Active (SM-G870A) is G870AUCS2DQD1, released on May 7, 2017 for 1 region (CSC). Every official firmware build for SM-G870A, 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/G870AUCS2DQD1
The newest build, G870AUCS2DQD1, was released on May 7, 2017. No new build has appeared for 115 months, so regular updates for this model have most likely ended.
Box codes of SM-G870A for different colors and markets. Firmware is chosen by the model code SM-G870A and your region (CSC), not by the SKU.

SM-G870F0

SM-G8750

SM-G975WC

SM-G889A

SM-G888N0
The latest build for SM-G870A, G870AUCS2DQD1, was released on May 7, 2017 (9 years ago).
SM-G870A has had no new firmware for 115 months, so regular updates have most likely ended.
Yes, all firmware files for SM-G870A are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G870A, 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-G870A.