I've been using Gotify for years for various notification use cases, and it's awesome. Specifically for the last 2+ years, I've been using it so that I can have Zulip notifications on Android, since the app requires Google services (FCM) for notifications, which I don't have on my phone.
Here's the thing I banged together to pipe Zulip events into notifications. There are a bunch of nice things I could add to it, but I haven't had enough need to do so yet.
I stood one of these up last year to play around with it. It works flawlessly but the Android app destroyed my battery life. If that has gotten better, it's probably a fantastic solution for DIY real-time push.
I guessing that is because it wants to work without google play services so it needs to keep a connection to the server. This is similar to how signal works when run without gApps.
I think the difference is more that each app has to maintain its own connection (more connections, more drain). Seems like gotify could remedy that though.
I've been using it for over a year now and it hardly uses any battery on my phone (Huawei Mate 10 Pro, Android 9, app is whitelisted from all battery optimization). It's so low in fact that it often doesn't even show up on the usage list, and if it does its 1-2%.
Wait, so setting this up will give me a REST endpoint where I can send data to, which then gets pushed to my Android phone? So that I can ship any kind of actions/events to my mobile? That's awesome!
I've wanted something like this for quite a while but instead I've been mainly using webhooks/bots to Slack/Matrix/Mattermost for this.
I'm a huge fan of gotify. I use it to receive notifications when my backup system does its job, activity from my bittorrent client, alerts from fail2ban, and I'm sure a bunch of other things I'm forgetting about. Since it uses simple webhooks, so long as you can execute curl you can use it to push notifications out. So handy!
If I see persistent attacks from an IP I'll step in and blacklist the host proactively by setting up an evergreen iptables rule. It's incredibly rare but it happens once in a blue moon.
As suggested below, Pushover is great (you have to pay couple of bucks). Another free option is using, surprise, telegram for sending notifications to yourself
You first get Apple to allow apps to keep persistent websockets in the background.
They recently added APIs for using a local push notification server, but you must necessarily be connected to Wi-Fi for it to work.
Here's the thing I banged together to pipe Zulip events into notifications. There are a bunch of nice things I could add to it, but I haven't had enough need to do so yet.
https://github.com/cyphase/zulip-to-gotify/