I like the way you address things, but where do you stand on the matter and why? I am just legitimately interested. (and tbh I kinda just also want to see you address another argument lol)
Sure, I can answer to this.
Depth is one of the core determinants that makes fighting games fun, challenging, memorable, and rewarding. It is undeniable that L-canceling adds significant depth to the game. Without it, then many combos and follow-up extensions would not be possible due to increased landing lag. However, what the argument is about (or what it should be about) is whether the
manual execution of this mechanic adds any depth to a significant degree in order to support having the mechanic in the game expressed manually as opposed to an
automatic execution of it (or as opposed to some other variant of the mechanic).
To answer this question can depend on the perspective in which you look at this issue and the things you value as the most important. I choose to look at this from the perspective of a game design standpoint because initially, L-canceling is a game design decision and I look at how this decision affects potential players and cross-reference this between other alternatives (auto-L-canceling, no L-canceling, etc.) and after doing this, it's extremely clear that manual L-canceling adds absolutely no depth into the game. Just simply ask yourself what depth is being added if it did add depth? From a game design standpoint, it forces the player to press an additional button to achieve an effect that they want to have happen 100% of the time anyway. Imagine if this was done for anything else. Imagine that anytime you wanted Fox/Falco's shine to come out on frame 1, you had to press an additional button, otherwise it would come out on frame 2. Imagine if you had to manually L-cancel every spacie short hop laser, otherwise you'd get significant lag from each attempt. Imagine that you had to press an additional button to make Falcon's Knee come out on frame 14 and if you didn't, it comes out on frame 28. In what situation would a player ever want the alternative? When designing a game, you want to be able to give the player reasonable options that can be used to their benefit in order to enhance the gaming experience. You also want such options to be reasonably accessible to the large majority of players and to only increase difficulty when the reward increases also, proportionally. This is why having a good skill floor is important for making sure the most number of players play and enjoy the gaming experience. When you set up a manual mechanic such as this where there is no reason not to do it, you limit the decision tree of possible options to one, singular option at the cost of increasing the skill floor with no measurable benefit to provide for it in-game as opposed to an automatic variant of the mechanic. Unless increased difficulty only for its own sake is used as a metric to add value to a game (which from a game design standpoint, it isn't due to artificially increased skill floor), that is the definition of bad game design. If you value game design as a way to measure the importance and degree of experiences that a game can provide, such as depth, then it logically follows that that which is bad game design is bad (or non-beneficial) for the game in question. Therefor manual L-canceling is a bad mechanic from a game design perspective.
Now, if you look at the topic from a perspective that puts more value on hard work, individual skill, and high skill floor and if you believe such things determine the worth of a game, then manual L-canceling becomes a very appealing mechanic that can be very easily argued to be a beneficial, positive one. When you artificially increase the skill floor, you instantly separate the men from the boys. You set up a scenario where in order to even participate in the higher levels of the game, you have to get over a technical hurdle that requires a lot of hard work and training for some. But once you get over that hurdle, it's as if one has climbed to the peak of a mountain or finished a long race and can then, from that point onward, reap the benefits of that accomplishment. It is very similar to the mindset involved in sports, which some people would argue that video games is a part of. The best of the best can only be determined (or is usually determined) by how much hard work and effort someone put into their sport and is summed up by all the hours of training, practice, repetitive exercises, etc. in order to achieve the end product which is the ability to even have a chance (even if it's a tiny one) to compete with the best of the best. Many people find much more value in this compared to some discussion about objective game design or something. Some people would even define such experiences as what it means to have a game with depth: A game that can provided these feelings of accomplishment through hard work, effort, and practice. Having such a mechanic automated is a kick in the face to such people and it seems like it automatically devalues all the hard work that they put in just to appease some "noobs".
I completely understand the latter perspective and understand how this makes people feel. However, when you make the value of a game mechanic dependent on things such as the value of hard work and individual effort, the discussion becomes subjective very quickly because emotions start to enter the discussion and are used as justifications for permanent changes in a game. The problem with introducing emotions as a way to objectively measure the value in a game is that emotions differ from person to person. Not everyone feels the same way about hard work and effort and overcoming a high skill floor. Some people value other means of judging the worth of a game, some that are just as subjective, some that are more objective (like game design), and perhaps a mix of both. And so people just end up bickering over the same things because they end up having different values at the end of the day.
But the original question to the topic still remains: "Does manual L-canceling add depth to the game?" And I think that if one wants to answer this question as objectively as possible, they need an objective method of doing this. I think game design theory is the best means of doing this (though I understand there are differences in opinion on how to best design a game, but there are consistent elements that still make it useful to use as a objective model), and as such I would have to put aside any emotional ties I have with the topic such as the personal satisfaction of L-canceling in order to reach more closely to the truth, which is what I ultimately care about in the end.