is there any software available for monitoring multiple websites for updates? If a update happen my raspberry should give me a notice of some kind.
What do you mean by “updates”? If the content of a website changes, a new post like in a blog or forum, or an update available for the underlying website software when you host it?
Best to my knowlage, DietPi did not offer a tool to monitor content changes on web sites. But there are a lager set of tools available https://www.guru99.com/monitor-websites-change-detection.html
Yes, such things I was looking for. What a pitty that this is not available for Dietpi/Raspbian because all these Online-Provider charge its service at the end.
If the website had an RSS/atom feed, you could use a news feed reader/aggregator daemon/app which usually have some notification system as well.
An easy custom solution could be to do a HEAD request and ckeck the last-modified header, which is mostly set, at least:
curl -sSfIL https://dietpi.com/ | grep last-modified | tail -1