mastodon.me.uk is one of the many independent Mastodon servers you can use to participate in the fediverse.
Open, user-supported, corporation-free social media for the UK.

Administered by:

Server stats:

500
active users

I am sure I saw this as a thing, but either forgot to take a note of the name, or had a dream about self hosting this (because I'm just that sort of exciting guy ;-) )

Is there a “messaging/notifications gateway” service, which I can run as a docker container, open a web page to configure different destinations (eg slack, discord, signal, whatsapp, mqtt, etc), then post messages to that and some logic (tags, client key)sends the message to my endpoint(s)?

MatStace

Looks like I'm using Apprise, with thanks to those who mentioned it, and not just because it also has APRS and DAPNET (hurrah ham radio stuff).
I suspect I might have conflated another web app and Apprise to give myself a false memory, but I'll update this if I find the mythical web app in my head.