I’m not sure if I’m missing something, but in those docs it does say about installing agents for RHEL8 and later. That would suggest it’s supported still.
I personally use CheckMK, but depending on what you want in terms of monitoring, another free option is Sensu. I looked at Sensu when I was also reading about Icinga, etc. I don’t think the free one includes the web interface, but I did manage to find a free one that was basically the same as the paid one anyway and it was OK.
CheckMK is pretty good, and Nagios plugins will work with it, if CheckMK is lacking some plugins that you require, but they generally have the majority of stuff covered. Personally I’m not a fan of Zabbix, I’ve tried it a few times, and I just don’t like it. But I know a lot of people do like and use it.