The origin of Sword Dao is the True Sword Dao.
Among the Thirty-Six Immortal Skills, True Sword Dao and Yuqing Daoji are the easiest to obtain. For example, to master the first three levels of the True Sword Dao, even if one is just a disciple learning the basics of the Yuqing Sword Skill, only an elder who has attained the Mahayana Realm is required. Similarly, Buddhism and Demon Sword techniques are not much different.
The latter, Yuqing Daoji, is even easier. The Sanqing Taoist Sect openly allows outsiders to learn it. Although they do not actively spread their spells, anyone with enough talent who goes to Three Pure Mountain will not be begrudged the teachings by the sect.
Like most skills of ascension, exceedingly few have managed to master them over the millennia.
The higher the level of the magic skill, the less tightly it is kept secret. This is simply because of the extremely high difficulty; sects already publicize teachings in order to find capable cultivators.