In previous versions, the block light level value on the F3 screen would read 0 when you're on a block that can't spawn monsters. Now they read normal numbers like a full block. This makes spawn-proofing more difficult since you can't just ignore the places where the block light level is 0, you have to check which blocks are bottom half slabs or other non-spawnable blocks, and which ones aren't.
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Just FYI, this was a feature in 1.13.2 (see file in main thread). Probably has to do with the light update.
I don't think that it was never intended to be a player-helping feature changes this, it's still a useful thing that has been removed.
It would seem you were correct, however it turns out that behaviour was in fact a bug, which had been fixed early in 1.14 snapshots. See MC-46667.
To my knowledge, it has always displayed light levels for all blocks. Keep in mind, these are used to determine not only if a location can spawn a mob, but also things like plant growth, and... you know... lighting. It has in no way been a feature designed specifically to help the player check if a mob can spawn at any given location.