Does the 20% - 80% rule apply to the ipad too?

The battery health feature is exclusive to iOS for iPhone - inclusion of this feature for iPhone being relatively recent.

iOS/iPadOS for iPad has never had the Battery Health feature - most likely because iPad batteries are both considerably larger and have a much longer service life.  An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles - whereas an iPhone is rated to have 80% of initial capacity after 500 full cycles.

A full charge cycle is defined as any combination of charging that together will total 100% (e.g., 50 + 20 + 20 + 10 = 100%). As such, reducing the number of full charge cycles is the key to extending the overall life of the installed battery; using the Power Adapter instead of relying solely upon the internal battery will reduce the need to recharge.

While your iPad doesn’t have a battery health setting, the iPad will automatically regulate and optimise charging - and will not overcharge the battery.  When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times. Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

In summary, it is generally unnecessary to take any special precautions to maximise the service life of your iPad battery. As already mentioned by Limnos, this Support Page outlines Apple’s specific advice for prolonging the life of your iPad’s Li-ion battery:

Batteries - Maximizing Performance - Apple

I wouldn't worry about it. Does the battery serve you, or do you serve the battery? If keeping it within a charge range was all automated (like in modern electric vehicles or some laptop computers), then OK. But it's a ridiculous amount of monitoring to try and keep it within a certain range without any kind of automated tool to do it for you.

The other thing is that what 0% and 100% are is chosen and not inherent to the battery technology. It's pretty obvious that iPad and Mac batteries are rated at 1000 cycles (as opposed to 500 in iPhones) because the electronics automatically keep the charge in a narrower range. Even at 0% there's still usable charge but it was a choice. Most electronics with lithium-ion batteries could theoretically double the usable charge if the user was OK with 25-50 full charges before it's depleted.

If the battery goes kaput, you have the option to get a battery replacement service, which is really just a full replacement. I believe it's now $119 plus tax in the United States for all iPad models, although it's only eligible if it's in good condition (with minor cosmetic wear acceptable). I've had an iPad mini 4 determined to be ineligible because the case was bent after being dropped. If there's any other kind of "damage" they'll ask for a full replacement price. I just got a warranty replacement (basically just a swap like a battery replacement service) yesterday, and before they would go through with it, an Apple employee placed it flat on the counter to make sure that the case wasn't bent/warped. I tdid have a lot of scratches at the port and there were some marks from being in a case, but they weren't bad and weren't disqualifying from a warranty or battery replacement service.

I believe the Battery Health app is on iPhones only. However, this article may be of interest: Batteries - Maximizing Performance - Apple

You are very welcome.