User should be able to select which platforms the message is sent to. Checkboxes should be enabled for all options by default.
See example below.

Send the announcement to:

• Discord
• Email
• Telegram

Currently sending the characters <, > and & will fail due to the HTML parser thinking they are a part of a HTML tag in the Telegram sending function.

They should be escaped accordingly: "All <, > and & symbols that are not a part of a tag or an HTML entity must be replaced with the corresponding HTML entities (< with &lt;, > with &gt; and & with &amp;)."

See https://core.telegram.org/bots/api#html-style for more details.

An alternative would be switching to MarkdownV2 parse_mode.

Currently messages are sent instantly to all selected platforms.
If the messages are send at night then people might ignore them along other night time spam notifications.
Scheduling messages to reasonable times (aka when people are awake) should be considered.

Cancelling schedules messages should also be implemented to prevent accidental messages or to make it possible to edit out spelling mistakes etc.

src/vendor folder is missing usually in situation where admin has forgotten to do a composer install. Advice to try it.

See Bootstrap alerts for styling: https://www.w3schools.com/bootstrap/bootstrap_alerts.asp
The warning div can be on full width of the page top, text aligned to left.