How do ETSOAs/TSOAs expire?
From the Code of Federal Regulations CFR 14

§21.613   Duration.
(a) A TSO authorization or letter of TSO design approval is effective until surrendered, withdrawn, or otherwise terminated by the FAA.
(b) If a TSO is revised or canceled, the holder of an affected FAA letter of acceptance of a statement of conformance, TSO authorization, or letter of TSO design approval may continue to manufacture articles that meet the original TSO without obtaining a new acceptance, authorization, or approval but must comply with the requirements of this chapter.

From the FAA's page on TSO Cancellation/Withdrawal:

What is the difference between a &quot;Canceled TSO&quot; and &quot;Withdrawal of a TSO authorization&quot;?
A &quot;Canceled TSO&quot; means that the TSO has been rendered inactive and the FAA will no longer issue new TSO authorizations against that particular TSO. However, any existing TSO Authorizations obtained prior to the cancellation of the TSO are still valid and may continue to be manufactured and marked in accordance with that authorization.
The &quot;Withdrawal of a TSO authorization&quot; means that a specific authorization letter is withdrawn by the FAA and the approval to manufacture that particular article is terminated.
In summary, if the TSO is canceled, you can still make the article but you can't apply for a new authorization to that particular TSO revision level. If the TSO is withdrawn, you can't even manufacture the article under the TSO system (i.e. the FAA won't let you put a TSO label on it).

An example of the FAA suspending a TSO Authorization NavWorx ADS-B