How does one determine Lead-Acid battery end-of-life?
The best way is to periodically (eg every few months or so) do a capacity test by running the UPS (from fully charged, with power off) with a known heavy load and recording the time it takes to drop out on low Volts.

The important thing is that you can graph the results over time (eg years) and note how it is deteriorating. This will give you notice well in advance of any failure due to low capacity.

If you can't do a full capacity test, simply measure the instantaneous voltage drop with a known large load, and again graph the results.

Terminal voltage by itself is not at all useful.