Can I add thin brick veneer directly to interior plaster
The keys are the "key". When real plaster is applied over wood lath, the base coat oozes through the gaps in the lath and sags or expands a bit forming "keys" that, when dry, hold the plaster layer to the lath.

This is a strong structure unless a substantial amount of the keys have been broken (from temperature or humidity swings, structural shifting, physical damage). When this happens in a specific area, that section of plaster is basically unsupported and is prone to cracking and buckling inward.

If the plaster is generally sound, you should have no problem. While drywall is more flexible and less prone to pulling away from the framing, sound plaster should be strong enough to hold the weight of a thin veneer.

If, however, significant sections of the plaster are bulging outward, or if there is a large amount of the surface coat pulling away from the base coat (a sign that the plaster is in poor condition overall), this is not a good basis for a top coat beyond paint or paper. In that case, you may need a more significant redo.

If the weak areas are just a few and not too large, there are specialized plaster washers that are used to screw the weakened section to the lath to prevent further damage. That may be sufficient if the areas are few and far between.