Back then 20+ years ago, a piano patch is generally quite difficult to come by. What we have are the FM stuffs which give so-so piano patches. Maybe some PCM patches - that's about it. Digital keyboards are just about growing and coming up replacing the analogs. If you have a keyboard with a "good" piano patch, it probably cost in the range of a fully-loaded 88-key workstation today (relative pricing comparison, not absolute). All I recall is that the Junos were the most common ones we could find around - and they were good! DX7 came up 23 years ago and for a while was one of the very few good digital ones that were around at that time.
So it's really hard to compare what we had 20 years ago - technology has improved and therefore cost and availability has changed. Today, you can't find a digital keyboard without a piano patch. With relatively low cost, a studio should be able to get a RD300 or the older RD150 with most of the bread-and-butter keyboard sounds. But with the improvement of technology, we keyboardists' taste-buds also changed. No longer do we just look for "piano patch" - we want it to cover most of our needs from analog to digital, from acoustic to electronic, with good touch and response and preferably master controller capabilities so we can have the option of bringing extra outboard gears and hook up by midi. I don't frequent studios but I've seen a number of studio's websites who posted their gears. Comaparatively speaking with 20 years ago, studios still don't have the high-end keyboards we wish they have. I guess everything revolves around cost.
Iansoh mentioned Boon's studio having RD600 and a Triton - I'm actually quite amazed and impressed! I wonder if that's the norm... I doubt it though. Perhaps somebody can prove me wrong here.