Apologies -- I had no idea anyone was using that function. It was only in ZLib for two months, and replace_string() can do what it did, but better. Weeks ago, I simply edited it to what flypapermonk posted above, but I figured only one or two people might need to edit their scripts, which would be easy since I was sure the function was not widely used. So in the interest of optimal scripting and streamlining ZLib, I removed it.
Needlessly Detailed Reponse
(skip to the summary if short on time)
I promise that I will only remove functions if something far better is added to ASH. It is my philosophy not to have just functional scripts, but optimal ones. On the other hand, it is also my philosophy (as evidenced by just about everything I've posted/shared/made) to provide useful, easy-to-use scripts and scripting resources for the mafia/scripting community, with a minimum of inconvenience. So, while I strive to make ZLib of maximum usability to all, I view one such use as promoting script optimization.
In this case, replace_string() is superior in every way to the old ZLib function. Though you may not like having to replace it, it's for your script's own good. I feel this is akin to when the change was made disallowing = (requiring ==) in conditions. Most everyone had to track down these places and fix them, but it was better.
Here I simply removed it due to its short lifespan and very specific application. However, if an ASH equivalent to obtain() were to come along, I would simply make the existing ZLib function a wrapper for the new ASH one.
Summary
Don't worry. I am very considering and cautious about changes to ZLib, since I know that now (surprisingly enough!) quite a few authors are using it.