what are biggest dagger/lee/centerboard ever used?

These seem to have sorta gone away the bigger the sailing ship gets. I'm sure most of that is due to more limited structural factors for RELIABLE large moving parts in olden days when you couldn't call for rescue.

Whats the deepest normal deep sailing keel ever used?

Any reasons, in this day and age, any reason why a really, really big (like 10,000 tons or more, with dagger or other boards 100ft deep) sailboat/sailing ship couldn't have a fully retractable dagger or centerboard (including MAYBE two side by side like bilge-keels to keep individual board size more manageable) so the vessel still has reasonable draft when retracted?

Maltese Falcon has fairly small what looks like centerboard. Specs on Wiki say draft is "20/36ft".
Maltese Falcon yacht model https://www.modelshipmaster.com/products/Super_yachts/maltese_falcon_superyacht.htm

 

There are a lot of reasons, both structural and hydrodynamic, why you don't want a board that big or that deep. FWIW, the draft of most fully rigged ships by the mid 19th century was 18-22 ft, which allowed them to get into most undredged ports worldwide. Today, merchant drafts are typically 30-35 ft with 40 being about the limit (due to the 41.2 ft sill depth of the original Panama Canal) to prevent massive infrastructure problems though that may change with the new canal locks. If you look historically, the largest centerboard/leeboard sailing vessels were US schooners and Dutch barges respectively, for the Chesapeake and Zuiderzee respectively but needing to be offshore capable. (Yes, I know about Humber Keels, but they were size restricted)

 

Some of the Spanish and English first raters drafted about 30 feet.

Nelson's Victory, 1759, drafted 28'9".
Brueys' Orient, 1791, drafted 26'8"

 

They probably needed every inch of it, and the ballast that came with it, to be stable with all those guns. Never mind be a good gunnery platform.

 

Both true, but remember that Victory came in significantly deeper draft than expected with ~4 ft or less between the lower port sills and her normal waterline. This makes her just a big clumsy frigate in SS3-4/Beaufort 4-5. There is a reason Nelson preferred Captain.