Invisible light source 1.12.2

You can hide it under carpet or behind a painting. Maybe a banner but haven't tried that.

Unfortunately no.

Light blocks? Idk if they're in Java or Java 1.12...

is there maybe like a data pack that can do that>?

Depending on where/what you are building, you could either use end portals or retexture a block with a resource pack to be completely invisible.

End portals can be placed out of a frame with /setblock, emit light and are only visible from above, so if you put them against the ceiling the player can not see their top and they are invisible. Problem is, they still send anyone that touches them to the end so to be completely safe you would need to put barriers under them and make the rooms they are at the top of tall enough players can't reach them. Because they are lifted so far up and out of the way, the light they cast may be slightly dim by the time it reaches the floor of the room.



You also can't have them anywhere players can see them from above, or they will be visible from that spot.

This is a command and it might be what you are looking for.

https://www.youtube.com/watch?v=_ALwqBXbN9k