The method of monitoring the presence of AC 230V input voltage, described in the article does not work with 1 Gbit ports:
https://mikrotik-ukraine.blogspot.com/2016/06/220-e-mail-sms-mikrotik-apc-smart-ups.html
Mikrotik router is powered by a small UPS without communication ports.
What is the simplest way to transmit information to hAP ac3 about the presence/loss of AC voltage?
Most simplest way i think use 4 diode (for complete isolation) with 2 different voltage adapter, like 24v adapter connected to main and 12v connected to ups. Combine pins with diodes and power unit from dc jack. When your mains fail you can see that /system health voltage level drop 24v to 12v. Unit work without interruption.
Or maybe you can use dc jack with poe in without diodes. Idea is same.
I did the tests for router hAP ac3 when powered for two cases
1 - 24V AC adapter only,
2 - powered only by PoE in 15-24V.
command in the terminal #system health print = empty out (nothing)
for example on router hEX
system health print
voltage: 24.3V
temperature: 36C
HAp ac3 does not have a supply voltage meter.
In the article you cited everything works with 1Gb ports too, read the comments at the end.
You need to disable auto-negotiation and leave support only for 100 mbps
Didn’t check it. My bad. If empty port available he can connect cheapest dummy switch to it with main connected and monitor port status if running or not. 
Results of my practical tests after manual port transfer to 100 mbps - the link/unlink works only for 1 minute.
After 1 minute there is no more reaction of the port to the external link/unlink.
ARP disabled, Loop Protect default.
Maybe useful (pinging a loopback cable on given port):
http://forum.mikrotik.com/t/sensors-how-to-make-a-door-or-other-type-of-a-sensor-to-monitor-things-such-as-open-doors/146520/1
for ping you need a locked port, but now it doesn’t work for 1 gpbs
On the hAP ax3 router #system health print - also only CPU temperature without voltage!
Why is there no supply voltage monitoring?
Is it possible to install additional power monitoring circuitry in these routers?
For example, via a resistive divider to the free processor input.
Mikrotik specialists, please advise.