Re:"...is it mandatory to remove the battery..."
The way you worded the question made it difficult to answer. If I take "...is it mandatory to remove the battery..." literally, I would have to say there is no law or regulation I know of that mandates you to remove the battery. But I think you mean "does it do harm not to remove it", the the answer would be it depends.
LiIon battery will be damaged when it over discharges. A battery left on its own will self discharge. A battery typically have protection circuits. When off, that protection circuit may still take a little leak from the battery so a protected battery may self discharge a little quicker. When that battery is connected to the phone, the leakage may increase and the battery will self discharge a little quicker still. From what I have read, it would be equavlent to > 1 giga ohm, but leaking a pico-amp leak is still is still a leakage. So, the answer would be, well it depends, but likely the less the battery is connected to, the better off it is for the battery.
How much and how long it takes for it to self-discharge to a point where it harms would depends on too many factors to have a definite answer. The longer the battery is at a high voltage (keeping fully charge) will also lessen the life of a battery. Charge to 80% is the typical answer for battery going into storage, and recharge before it reach 20%.
Until someone can post some better stuff (love to see somelab test results), here is some personal experience to give you gauge on loss of battery capacity:
(1) I had four 18650 LiIon (with protection circuit) that was charged full when I was about to out it to use, but went into storage by mistake - it was misplaced then went into storage when found. Three to four years later, only one went down below 4V to just under 3.8V, well above the 2.6V floor. Charge/discharge tests shown that the batteries' capacities were still above nominal capacity so absolutely no significant damage. I can't say if it will take another two to three years before it drops to 2.6V, but three to four years certainly didn't come even close to doing it.
(2) I purchased a couple of new-old stock older and low-end model smart phones - probably two to three years old at time of purchase (market was selling Andriod 7, these were Andriod 5). They both power up and ran without problem with self test indicating battery is in "good condition". One was in use daily for about two years now as my "quick" internet browser and battery apparently was holding charge quite nicely. So, two to three years of storage with merely phone-off as the sole battery saver and no apparent issues.
Re: "...difficult to remove without damaging for a person who has little experience..."
Yeah, you have to do it very carefully. I've replaced the LCD on a smart phone, once for myself and once for my kid - different model but same manufacturer. The battery needs to be removed before I can get to the LCD. Getting the battery out is probably the hardest part with those models of phones. The tape-glue is awful to deal with and yeah, it sticks like glue. I was afraid the outer wrapping of the battery will tear as I pull hard to get it off the tape.
If the video you linked to is not the same model as yours... It will serve you well to see if you can find a video of others doing battery replacement for a same model phone you have. Also, you have to be very organized to make sure you know how to put it back together.
Use soft-tools. Very hard tools will scratch and can cause damage like cutting into the battery itself.
I ran into a situation that I couldn't remove the battery because of hidden glue-tape between the battery and back frame. Had I use something like steel blade to ply it or cut it, I could have damaged the battery. I was well aware of that potential problem. So, I cut a piece of plastic from a 2 liter plastic soda bottle. The piece of plastic has no sharp edges of any sort - I even rounded the corners well. It was soft but hard enough to be use as a plastic knife to cut the tape in between the battery and the back frame.
It is best to use tools that is just hard enough. It will minimize damage should you make a mistake.
Lastly, a different suggestion:
Leave the battery as is and just rely on phone-off, but check it regularly.
- Find the app call "CPU-Z" that does phone diag. Use it to check the battery.
- Take a screen print. Now store your phone with just power off and mark the calendar.
- Check it every 6 months and monitor the progress. See what is the current battery health (good? fair? bad? whatever.) as well as the voltage. If the voltage is down to 3.0, time to give it a recharge.
In a year, you can share that with us and we can all learn something as well.