A small clarification before I wrap this up:

_Description: Use APT data sources from Matrix.org?
 The APT package management system may be configured to download supplemental
 Debian/Ubuntu packages via the Matrix.org package repository. These packages
 are digitally signed with keys from matrix-archive-keyring, but may require
 software outside of your system's distribution to either build or function.
 .
 Packages from these sources may not be up to par with the technical
 requirements set in the Debian Policy for this operating system.

That dialog only triggers for Debian and Ubuntu, I'm afraid.

But I agree now, I shouldn't say that because they are not part of the Debian or Ubuntu distributions

Good point. Changed it back, thanks.

I'm tempted to change

Trust this keyring in APT to authenticate packages?

to

Live dangerous(ly) and trust this keyring in APT to authenticate packages?

_Description: Trust this keyring in APT to verify data sources from anywhere?
 You may choose to enable matrix-archive-keyring as a system trusted keyring
 for the APT package management system, if you need it in place of the Debian
 archive in some situation.
 .
 Adding keyrings to APT's list of trusted keys is not necessary in most cases,
 and may be a risk for your system. Usually, it may be simply enough to use the
 "Signed-By" option described on the sources.list(5) manual page.

Ended up with this.

New build. Changes:

• If you have configured /etc/apt/sources.list.d/matrix-org.list manually before installing this package, the debconf(1) question "Use data sources from Matrix.org?" will default to "yes" to use them (instead of always "no").
• Code cleanups. "Bad APT keys" warning should show up much earlier now before configuring the package, rather than at very late after the package has been installed.
• Minor changes to wording of debconf(1) templates (questions).

Ping Maximus. Thanks so far.

With #1, I mean migration from old Synapse install instructions will be smoother.

Btw, if you want to build from source: Extra the *.tar.xz file somewhere, apt install devscripts then run debuild -us -uc in the directory to create the same files as attached above to the ../directory.

ok so I would like this to happen:

• remove the pgp key from the global trust
• have the key added to its own keyring
• have the apt source only with that keyring.

I managed with the previous package but this didn't work (this is a debian system)

I guess. If says "no" if it didn't find /etc/apt/sources.list.d/matrix-org.list (that filename specifically), or if you had previously chosen "no".