Pages has never had its own Thesaurus, but rather allowed access to the Thesaurus in the Dictionary application — if it was selected in the Dictionary application's Preferences — by means of the optional three-finger tap, control+command+D keystroke sequence, or Look Up on the secondary menu.
The drag/drop order of your selected, Dictionary application's dictionaries (see its preferences) will determine the dictionary results displayed in the Look Up display panel (shown below). If a particular word is not found in a specific dictionary, then that dictionary will be skipped in the display.
Here is the order of my dictionaries in the Dictionary application:
For the word knowledge, in Pages v7.3, and based on the dictionary order from above: