This information isn't secret so here is what I could gather.
How many coded rooms are there in Zalanthas?
Currently, 31023. I believe this is only rooms that are accessible by players.
How many objects?
How many mastercrafted objects?
Currently, 5961. Though these are craftable objects many of which were mastercrafts and many of which were made by staff members and builders over the years.
How many NPCs?
How many player-characters have died since the game began?
This information isn't technically since the game began, because things were recorded differently early on but since we have been recording status in the DB the total is: 40,234
Also, the stored number is: 6,791
The currently alive is: 10,856
Note: These numbers are only mortal characters, not staff accounts.
How many total hours of playtime on all accounts? (Scary question, I know.)
I figured out a way to get this information. It may be ever-so-slightly off but here is the amount of played time across all accounts (since accounts were a thing) as of earlier this evening:
838 Years, 11 months, 26 days, 6 hours, 24 minutes and 9 seconds.
How many individual tastes have been coded?
Bit of a tricky question. Between cure, food, drink and herb taste messages there are 142. This is slightly misleading though because some of the drink messages are duplicates but technically they are their own taste message.
What you didn't ask but should have:
Number of areas/zones booted into the game: 59
Number of helpfiles: 1295
Number of skills (some not in player skill trees or not completed): 535