My Apple Pencil (1st gen) stopped working on my iPad

Given your confidence in the Pencil battery status, here is one of the better troubleshooting guides for Apple Pencil:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

Assuming that you do not have any other iPad charging issues - and that the Lightning Port of your iPad is free from debris - whilst the Pencil is plugged-in to the Lightning Port of your iPad, verify that the Pencil is recognised:

Settings > General > About

Scroll to the bottom of the page - and you should find either one or two entries for the Pencil. Finding these listed entries suggests that the Pencil electronics are good - otherwise, you have a totally dead pencil.

Check the Battery Widget for the charge status of the Pencil. Is it 100%? If not, let the Pencil charge; if it is fully charged, disconnect from the iPad. 

When disconnected from iPad, does the Pencil disappear from the Battery Widget - or it’s indicated state of charge suddenly fall? If either of these are true, then the Pencil battery is likely to be failing or dead.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.

If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced by your retailer or at an Apple Store.

I hope this information proves to be helpful in both diagnosing and resolving the problem.

Try a force restart. Also, unscrew the pencil nib and put it back on-make sure it is not lose. Most of the times these 2 things fixes the problem.

Good luck!

The apple pencil gen 1 is almost 7 years old. Chances are the battery died. You can try a force restart of your ipad and then unpair the pencil and try again. Do the force restart first:

https://support.apple.com/en-gb/guide/ipad/ipad9955c007/ipados

Regarding your other question regarding restoring the older ios- you cannot unless its a beta.