We have been playing in a "Game of Thrones"-like world where there is no way to learn magical abilities and people disregard its existence completely. But, I would like to insert some plot elements that would be obviously magical (a lord discovered a way to summon abyssal beings and bend them to his will). Do you think that would be ok and how do you recommend proceeding with this idea without losing the "normal people" feel in the environment
I think if you make sure that it's limited to one person, it won't feel too bad. As a player I might feel cheated out if you suddenly introduced a whole kingdom of magic users, but things are different if it's just a single person.
First of all, it would make for a great hook, you might even allow your players to absorb very limited powers when defeating him or his minions. Possibly not traditional magic abilities, because that would go against the heart of the campaign setting, but stuff like a magic sword with fire, or possibly some demonic vision ability of sorts, might make them feel special and accomplished.
Second, it would really help making the antagonist (I assume) feel very unique and powerful, which always makes for great sessions!
I would advise against asking your player's if it's okay with them, because I really think the most interesting part of this plot would be how mundane characters act, when suddenly faced with magic and unknown powers. I would also advise against giving any other NPC magic powers, even the antagonists followers and most trusted adversaries.
Last but not least I would highly suggest that you make everyone in your world that has experienced this be anxious, nervous, paranoid or aggressive towards the subject. This will help make the players feel like they aren't the only one's who suddenly have to adapt to magic and should help keep the setting mundane.