Demacia is anti-magic yes, but that doesn't mean there are exceptions to the rules that people are willing to begrudgingly tolerate or turn a blind eye to entirely.
Xin and Shyvana are Prince Jarvan's personal retainers, so obviously, they have given him an understanding that magic isn't bad or all evil like the general society believes. Therefore, these two people are under the direct protection of the royal family. Jarvan's father may feel shamed or disgusted by his son's choices, and the populace or nobles may despise Xin and Shyvana, but they are locked by decorum and propriety in how they can deal with them because of the prince. They cannot openly disrespect the Prince, and Jarvan's family cannot openly chastise or punish him without becoming a public spectacle of familial unrest that could put blood in the water and draw the dynasty's enemies to find weak points to try to take the throne.
As for the Crownguards and their latent magical abilities, everyone but Lux lives in constant denial. The only reason Lux doesn't is because she's worldly, and her magic is too much to simply attribute to divine intervention or the works of deities like you could apply to Garen's magic.
Galio is powered by petricite, and as such, he's a pure creation of Demacia. His life when exposed to magic is probably not even considered magically based, simply on grounds that he's a weapon of war created by Durand to protect Demacia. Most people are probably even too ignorant to know how petricite works or even how Galio comes to life, so again, most probably live in denial about how he functions. And really the only person in Demacia who would have lived long enough to see Galio be alive more than once (or at all, outside of Lux) would be Poppy, as Yordles are very long lived.
