The context really matters with defoliation. Indoor growing in a scrog or any flat type of canopy it seems beneficial to remove lower leaves around the time of stretch. How much up the plant you remove depends on the penetration of your lights. For 600Whps If I recall, you can remove anything that is below 1 foot from the ''flat'' canopy top. With led or cob you tend to see top shelf stuff within the first 6-8 inches. Removing upper leaves to allow light down below the optimal reach of your lighting choice is completely counter productive, never do that. The occasional leaf here and there sure, but very occasional. Most of the reason for this type of defoliation is linked around light penetration, you don't have it.. so why run the risk of keeping extra foliage that will require more water/nutrients and increase pest/disease chance. It's got nothing to do with increasing yield, defoliating reduces yield, but subjectively we don't count larf or slightly less mature bud as top shelf yield. If defoliation is increasing top shelf yield you are doing something wrong in the first place, while I don't even buy into the concept that it increases yield even in that circumstance.
If you have a leafy strain then you need to account for that, correct spacing between plants but also the correct topping and opening of the plants at the right time. This better ensures you don't get mass amounts of leaf over lap. Mass leaf over lap isn't a reason to defoliate, it's a reason to re-asses your over all growing practice, including veg duration and humidity control.
If you don't want to top/train your plants the Christmas tree shape is already the best you are going to get (in that context) for light distribution, removing leaves is pointless. What you should do, is adjust lighting position so that it is closer to more of the leaves. Drop the light between the row of plants as best you can. This only applies to bulb growing with open hoods that you can manipulate angles with. Led/cob you could achieve the same but it's not worth the hassle... easier to go scrog style at that point. but if you insist on xmas style plants, then you are going to have to customise angled cob/led set-ups to get more surface leaf area within optimal light range. You can try SOG and keep lights directly above, but all the plants will have to be flipped/stretched at a height that is within your lights optimal reach, or else you lose leaf/light surface area.
Some reasons not to defoliate are as follows. Leaves block light down to the soil/medium. If you defoliate too much, light gets past and you allow shit to grow on the soil that would not have. While algae isn't much of an issue, other things are, especially if over watering.
With fewer leaves you also have less heat buffer between canopy and root zone. This means upper and outer soil volume dries quicker, potentially killing off parts of the root system. This does depend on watering frequency but it's a very possible issue.
Leaves also act as an amber light, simplistically. Having lower leaves can give early warning signs that you can catch, before upper more important leaves are effected. During late stretch and a few weeks after, lowest leaves also start dying off. This could be due to lack of light, but also as a sign that the plant needs more nutrients and is taking from the lower leaves. Those leaves can then be considered as a more natural ''boost''. If you don't have a fine grip on your nutrient ppm ratios and you remove a lot of leaves, you lose the buffer effect. I leave a set amount of lower buffer leaves on purpose, so that the plant can take them if it needs them and also as an early warning.
