[updated to correct my own error here, apologies for the inaccuracy]
Although the track parameter optionally did work with user streams, it also operate with the regular streams available via statuses/filter, and this will continue to work.
There is no way to replicate this with the Account Activity API, i.e. it is not possible to track Tweets from arbitrary users on top of the account activity messages from your subscribed account, into the webhook. If you need this functionality we would suggest that you look at using statuses/filter, or to investigate the enterprise PowerTrack product for even more features.
