No, the Sparks function has not disappeared in Google+, but indeed it is a bit more difficult to find it now.
To use the Google Plus sparks you must first search for your interest in the search box of Google Plus.
A new window with the results will appear and under the search box there is a new button: Save this Search.
Click on it and your search will become an 'spark', it will appear in the left side bar and when you click on it it will display your interest in the same way as before.