Is it OK to keep my MacBook Pro 2020 plugged in all the time due to Apple's battery management software?

I am really sorry that you had a bad experience with an Apple MacBook Pro battery.

I am even MORE sorry that there is an Apple technician spewing outdated nonsense that contradicts Apple's current stated policies.

Batteries are a 'wear item', like the soles of your shoes and the tires and brakes on your car. Eventually, they are likely to wear out, and are expected to need replacement.

Apple makes very good batteries, and early replacements are rare. But there is still a great deal of variability in exactly which battery lasts and which does not.

You are reporting on your experience with ONE Apple battery. That is not a large enough sample to draw ANY conclusions based on your experience That is FAR too small a sample to make blanket recommendations that fly in the face of Apple recommendations.

if this were a $10 toy, it might be expect to not have a charge limiter, and could conceivably self destruct if charged too long.

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That blocky device that plugs into the wall is the POWER ADAPTER. I am not trying to be a jerk about terminology, what these boxes do and how they do it is essential for understanding how they all work together. The LEDs in the MagSafe end are controlled by commands from the computer.

The CHARGER is inside the MacBook Pro, and is completely under program control. Your Mac accepts some power when needs to run, and accepts more power when intends to charge its internal battery. A power source like the Power Adapter can not 'force itself' on your MacBook Pro. Only the algorithms inside your Mac can decide when is a good time to charge the battery.

Despite your bad experience, the overall charging philosophy Apple advocates still stands:

In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Your Mac maintains optimum battery charge levels under program control, and will NEVER over-charge. 

Activate Battery Health Management and do not spend another moment of your time thinking about charging.

you can but it may not be the best for the battery and doing this will degrade your battery health. If its just mainly in one location and always plugged in you might as well buy a mac mini or a imac.

Your MacBook is a battery capable device. Leaving it plugged in 24/7 will not harm the battery as the software manages the device. Your battery will still charge when needed and discharge *** needed when plugged in. For example, my MacBook Pro has been connected to the mains for very long periods of time before being used remotely with no issues.

Batteries - Maximizing Performance - Apple