If you are taking out the salt be sure to do so gradually. Going from lots of salt to no salt all at once is a shock to the fish. Each
water change add a bit less salt to the new water until finally you are adding none at all.
It's true there are varying views on salt. I have not made up my mind about it. Most fishkeepers just don't use it cause it's not necessary and it is just one more thing to hassle with.
While it is true salt can be very harmful to catfish (such as Plecos) and other scaleless fish, I doubt a bit of salt would instantly kill a Pleco. They are really hardy. Unless you add a TON of salt to the water there was probably another reason for your Pleco's death.