I should have asked before, but are you certain the cylinder is leaking, and not a line, coupling, or connection? Can you provide a source for a pic of the actual part? I found some pics, but they were not all the same. I cannot confirm which part is what you have. Some of them looked like they could be repaired. The Sachs number you list is different than what I listed, but I found both, among other parts that were obviously wrong. They don't appear to be the same part, by the pictures I found.

I didn't find the GM part number to be discontinued. It was only unavailable with no known date of restock, or simply not listed. Discontinued parts are almost always listed and labeled as such. The part may be discontinued, but none of the sources I found showed that. Also consider, this part was never a GM part, but had a GM part number because it was used in a GM vehicle powertrain. Your truck and powertrain series is 25+ years out of production, which is one reason I suggest not looking for a GM red herring. That powertrain was superseded after 1998 (Cat 3126), but the general powertrain package may have continued on with many of the same parts, including the clutch slave. Any heavy duty service or dealership shop would know how to source the part. They may also have unique access to salvaged parts. A used part that doesn't leak is better than what you have now.