Great question, Andy!
Some games are easier to include than others. For example, when you say “Hoyle” I immediately think of the first three in the series which I played a lot. Definitely Sierra classics. But many more titles were eventually released and to me they aren’t really an important part of the Sierra we know and love.
As far as other studios like Impressions Dynamic and Coktel Vision, Collector and Josh are spot-on. They should definitely be included. Yes, they were acquired assets but Ken acquired them because he knew they would make outstanding games and be good enough to be under the Sierra umbrella. And he was right 🙂
It’s hard to pinpoint a “cut off” date for games to include on the site. I’d like to say “Chainsaw Monday” (Feb 22, 1999) but if memory serves me, Sierra’s final masterpiece, GK3 was released after this date.