Leaving your laptop plugged in all (or most) of the time

@Unhappy Canadian  A fantastic resource for learning about batteries is BatteryUniversity.com.  The "Learn About Batteries" link in the header takes you to a list of excellent articles.  The one most relevant to this discussion is "BU-808: How to Prolong Lithium-based Batteries" (link).  It explains why capping the maximum charge level below 100% and avoiding full discharge are beneficial to the long-term life of the battery.  The former also explains why it can be useful to avoid storing a battery at full charge.  In fairness though, the article does say this: "Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling."  My viewpoint on that, partly based on other resources I've read, is that BOTH keeping it topped up AND exposing it to high ambient temperature may well be a problem.  But if you don't have to deal with high ambient temperature, then achieving any meaningful reduction in the amount of time a laptop battery spends at 100% charge would require you to cycle it so many times that the additional cycles would impose more wear.  The equation may be different when dealing with a battery where the device isn't used all day every day and/or allows more runtime per discharge, but when dealing with a device like a laptop that is used most of the day for many users and only gets a relatively short amount of time per battery charge, I don't see deliberately running on the battery when you could be staying on AC power being a superior strategy from a longevity standpoint.  But that article explains exactly why setting maximum and minimum charge levels is beneficial.  Limiting the maximum charge level to 80% reduces the voltage in the cells, and setting a minimum charge means that most of the time the battery will simply be idle, neither being actively discharged nor being kept topped up.  Even only doing the former would be an improvement over the default operation if having the minimum charge setting might risk too much of an inconvenience for everyday usage.

@Unhappy Canadian Yes, it is my understanding that running on battery power unnecessarily is not a good practice, even if the alternative would be to have it kept topped up all the way to 100% (e.g. because the user might not want to limit their charge capacity or might not be aware of that option, etc.)

Number of cyclings is indeed a key factor for battery lifespan, but as explained in that article, exactly what constitutes a "cycling" is somewhat ill-defined.  Cycling a battery between 80% and 60% X number of times is not going to impose as much wear as cycling it from 100% to 10% X number of times.  If you use your batteries infrequently, then your cyclings will last you a longer amount of time because you'll have fewer cyclings per day, week, etc.  And if you have less "intensive" cyclings, then you'll get more cyclings. (Although here again, what constitutes a cycling makes that sentence a bit tricky.  For example, would spending a few minutes on battery running from 100% to 98% count?)

My own laptops spend the vast majority of their lives plugged into power, and I use the settings I specified.  When my older XPS 15 9530 reached 6 years old before I replaced it, I still had about 2.5 hours of battery life under typical usage, which for that system wasn't bad at all -- and was very good considering its age.

As for Apple devices, I can't speak to their laptops, but for their other devices, there isn't currently a way to customize their charging behavior.  However, with iOS 13, they introduced Optimized Battery Charging.  The idea there is that when you plug your iDevice into a power source to charge it, such as at night, it will initially charge only up to 80% and stay that way for most of the night.  And it will only charge up that last 20% shortly before they expect you to disconnect the device in the morning.  I've seen this used on my iPhone, iPad, Watch, and even AirPods.  More about that here.