The Order of the Lamp
There is still an interface graphic for asking about and joining the Order of the Lamp (LAMP00I0.IMG), and the following excerpt from the TEXT.RSC gives a little information on what it was all about:
We of the Order of the Lamp are the protectors of the Mages Guild. We follow the Palantinus, under direct orders from the Archmagister.
We are given appropriate rights to certain services at the Mages Guild. One's rank within the Order approximates ones rank within the Guild, in terms of rights and priveleges.
The Order is only open to those who have already demonstrated fighting skills and are in good standing with the Mages Guild.
The Order of the Lamp thus relates to the Mages Guild as the knightly orders relate to their respective temples. Maybe it was dropped because it would have made it too easy for fighter characters to gain access to the services of the Mages Guild.
Other orders and guilds you were supposed to be able to join but later couldn't are the Necromancers and the Prostitutes, which were completely written out of the game, though they still occupy a reputation slot.