Personally I hate Slack, it never works on my PC because of their organizations/workspace system, where you need a separate account for each community you are in. Also, it's really buggy and login often doesn't work, or switching accounts breaks things.

Telegram has a really good mobile app, and their BotFather makes it easy to create custom integrations. They display nicely a lot of notification formats (text/html/markdown) and it's free.

Slack is too bloated to simply use for notifications.