While there are other platforms used in IDPA, the 1911 single stack used to be the most common.
If you "buy into" the whole idea of IDPA, course design is supposed to represent self-defense scenarios. The participants shoot the various stages for two reasons - 1) The fun of competition, and 2) To improve their self-defense skills.
Due to the bulk and weight of the 1911, many elect to carry something smaller, even though they use the 1911 in competition.
Others decry this practice, and the mantra "carry what you shoot" is heard.
Personally, I practice for IDPA, though I'm not what would be considered active. (I intend to become more active when I retire...) And I practice with a Commander-length 1911 in .45 ACP.
This is also what I carry, with a spare magazine.
Without actual self-defense experience to back it up, all I have is an opinion that this is not an obsolete or ineffective platform. And, that carrying what you practice with is a very good idea if you expect to be proficient with it under stress.