I found this a few days ago, and use it as alternative to #gotify(up) for notifications on my phone.
I get notifications for SchildiChat (Matrix) and notifications from shell. for example notify me about finished backup from my servers.
#ntfy has advantages over gotify. ntfy can send imagrs, audios, files and even click-events (open a link for example).
and the dev is very good in support and fast. we solved together my Huawei p30, which kills all connections on activating and deactivating the screen (stupid huawei). So the notification works better than #gotifyup...
Installation of Server is easy as fuck. Install deb-package and configure your webserver. Very, very good manuals on the Website. Once installed, the Same website is available with example links to your own installation.