How to find your MTD authorised software in HMRC - and how to remove authorisation
Finding what software you have authorised
Follow this link - https://www.tax.service.gov.uk/applications-manage-authority
You will need to log in using your Government Gateway (and in due course your One Login for Government account).
Taxpayers and agents
Taxpayers
A taxpayer can see what software they have authorised.
Agents (accountants, advisors, bookkeepers)
An agent cannot see what software their clients have authorised.
An agent will instead see what software their firm has authorised.
HMRC screens
If you have authorised untied, this is what you will see.

Removing authorisation
In this case we can see we also have TaxSplat authorised. If you don't use them any more you may want to remove them:

Missing log out link in HMRC systems
Important: The HMRC page does not have a log out button at the time of writing this article - search for "Personal Tax Account" or "Agent Services Account" and this should take you to a page where you can log out