It's integrated with Mattermost currently, seems to work pretty well. Could also set it up on another VPS, for example on Hetzner (which also has excellent pricing), could also integrate another alerting method such as sending e-mails, or anything else that's supported out of the box: https://github.com/louislam/uptime-kuma/issues/284
Oh, also Zabbix for the servers themselves. Honestly, if things are as simple to setup as nowadays and you have about 50 EUR per year per node that you want (1 is usually enough, 2 is better from a redundancy standpoint, since then it becomes feasible to monitor the monitoring, others might go for 3 nodes for important things etc.), you don't even need to look for cloud services or complex systems out there.
Of course, if someone knows of some affordable options for cloud services, feel free to share!
I briefly checked the prices for a few and most of them are a little bit more expensive than just getting a VPS, setting up sshd to only use key based auth, throwing Let's Encrypt in front of the web UI (or maybe additional auth, or making it accessible only through VPN, whatever you want), adding fail2ban and unattended updates, and doing some other basic configuration that you probably have automated anyways.
The good news is that if you prefer cloud services and would rather have that piece of your setup be someone else's problem, they're not even an order of magnitude off in most cases - though I'm yet to see how Uptime Kuma in particular scales once I'll get to 100 endpoints. Seems like at a certain scale it's a bit cheaper to run your own monitoring, but at that point you might still find it easier to just pay a vendor.
At the end of the day, there's lots of great options out there, both cloud based and self-hosted, whichever is your personal preference.