[QUESTION]

[bygohigo]

What is your question?
[...]
Trying to download the .deb package for Debian Trixie.

The instructions state...

Ubuntu / Debian

Since PEP668 adoption is no longer possible to install pip packages outside a virtual environment, instead use the provided deb package:

Go to the latest release page (https://github.com/bayasdev/envycontrol/releases/tag/v3.5.2)
Download the attached python3-envycontrol_version.deb package

I do not see the attached python3-envycontrol_version.deb package on this page.

Install it with sudo apt -y install ./python3-envycontrol_version.deb
Run sudo envycontrol -s <MODE> to switch graphics modes

[tur1ngb0x]

DEB package seems to be missing for latest release, till then you can manually download the envycontrol.py and use it.

$ command wget -O '/tmp/envycontrol.py' 'https://raw.githubusercontent.com/bayasdev/envycontrol/refs/heads/main/envycontrol.py'

$ sudo python3 /tmp/envycontrol.py --help