Why?

• Webhooks are well suited for alerts as alerts, like webhooks, are one way only.
• Posting into a thread is very easy with webhooks, you just append ?thread={thread channel id} to the URL. This would be very helpful with sorting the alerts. The auto archival feature of discord threads would also help with reducing the clutter that would be otherwise present on the discord server with multiple channels for specific alerts.
• Webhooks would allow for easier posting across multiple servers (in case the Minecraft server owner wants alerts to be sent to a different discord server in order for example to hide them from the discord admins or to once again reduce clutter in general).

What and How?

It would be useful if you could send alerts to a webhook URL instead of specifying a channel.

Are there alternatives?

Implementing thread support. This would, however, not help with the easy cross-server posting.

Checks

• I have used the search at least once to check if my idea has already been suggested and perhaps already implemented.

Webhooks are well suited for alerts as alerts, like webhooks, are one way only.

Alerts can already use webhooks, although by having the bot create it. Allowing to use webhooks without a bot is planned as part of making alerts a separate plugin.

Posting into a thread is very easy with webhooks, you just append ?thread={thread channel id} to the URL.

Threads aren't supported yet, #1217 - alternatively the expansion being completed would allow for this via the query parameter.
I did not know about being able to use webhooks for threads, thanks for letting me know about that!

posting across multiple servers

This is also possible, the bot can be added to multiple servers and the Channel option in alerts accepts a list, eg. Channel: ["global", "alerts"]