Anecdotally, I've been working with the SOAP API for more than 10 years now. In that time I've only ever seen one notification of older API versions being deprecated -
Knowledge Article: SOAP API Legacy Versions Retirement. That was for API versions upto v6.0. And note this isn't even going to happen until Summer '18.
However, as a rule of thumb, try and keep everything one a consistent API version within the last 3 years so you don't get a big shock if they one day decide to force things onto the newer API versions.
You might find that this recent API retirement is a test bed for Salesforce. I.e. Getting themselves setup with processes to retire older API versions. Identify affected customers, etc...
Of course, that is only conjecture on my part.
Update: Summer '21 with see API versions for the SOAP/REST/BULK APIs ending support for v20 and earlier. Lightning Platform API legacy versions retirement
Update: Summer '22 will see a further set of API versions being retired - up to and including v30.0. Salesforce Platform API Versions 21.0 thru 30.0