Firstly sorry for bringing this thread back to life, but I needed to leave the BETA programme and it was not happening, yes it is December 2024!
For anyone that has the same issue, I hope this helps.
Having been through support and been asked to remove the app for the first time ever (iirc) before reinstalling it:
Android/Play Store said I was in the beta programme but after removing and several restarts of the device and even a new device going to App Store in the hope of reinstalling the Monzo app … subsequent times I can’t install as there is no option to, it simply says:
Removing your account from the beta test program. You may want to uninstall the beta test version and install the public version.
That is what I was trying to do, in fact install ANY version as I cannot get anything done on the web interface in terms of pots, payments, transfers, cards, etc.
I did some more reading and finding old links, none helped until I found the link I used to join.
According to that, it says that leaving the beta can take HOURS. And after trying and waiting over 6, I didn’t know if it was actually going to.
Here’s what worked for me:
Rejoin the BETA - this happened very quickly, within seconds.
Once in the BETA I was able to reinstall the BETA app.
I could have left it there but it seemed a good time to resolve it properly.
The sequence of the following two steps seems to be what makes the difference.
With the BETA still installed:
At the magic URL, request to LEAVE the BETA.
This happened quite quickly.
Uninstall the BETA version of the app.
Restart the device - seems to be a good idea
Install the main APP.
Here’s the URL that helped:
https://play.google.com/apps/testing/uk.co.getmondo
Merry Christmas! 
@bea Not sure if you could communicate this to the support team as they seemed unaware.