m.room.server_acl

This event type controls which servers can access the room at all. Be very careful adjusting this.

https://matrix.org/docs/guides/moderation#banning-servers-from-rooms-server-acls

Note:

When sending the custom event via /devtools for the first time, click the Event so it says State Event and asks for State key in addition to type, which can/is left empty as usual.
Subdomains appear to be implied by specifying 2nd level domain. E.g. matrix.org appears to apply to *.matrix.org at least on Synapse.

{
  "allow": ["*", "mjolnir.matrix.org"],
  "allow_ip_literals": false,
  "deny": [
    "matrix.org",
    "disroot.org",
    "privacytools.io",
    "feneas.org",
    "glowers.club",
    "midov.pl",
    "kiwifarms.net",
    "liberta.casa",
    "nitro.chat",
    "nerdsin.space",
    "thisisjoes.site",
    "pp13.space",
    "evulid.cc"
  ]
}

Explanations:

matrix.org, *.matrix.org
- Very big homeservers promoting centralisation of users and power. Also big source of spam, poor moderation and abusive bot performing public logging of members-only rooms.
matrix.org, evulid.cc
- Run instances of matrix-public-archive which misses proper ability to opt-out
nitro.chat
- Poorly protected from abuse, abuse contact joins room that their users are raiding and leaves immediately without doing anything.
disroot.org, privacytools.io, feneas.org, liberta.casa
- Legacy servers that have discontinued, are in progress of discontinuing or are just closed for public, that have PL100 in my rooms.
glowers.club, midov.pl, kiwifarms.net, nerdsin.space, thisisjoes.site, pp13.space
- Connected to abusive raids on communities that I am a part of, sometimes even moderate.
Others
- Most likely inherited from Community Moderation Effort.

Allowlist

This configuration only lets servers I or especially close friends have accounts on in.

{
  "allow": [
    "pikaviestin.fi",
    "mozilla.org",
    "tedomum.net",
    "pirateriot.net",
    "tchncs.de",
    "fedora.im",
    "jae.fi",
    "converser.eu",
    "artemislena.eu"
  ],
  "allow_ip_literals": false,
  "deny": []
}