Open tagging has been replaced with a new system in v5.

If you would like to discuss this, or get help on preparing for the v5 upgrade, please use ourfeedback forum.

What needs to be done?

Nothing really needs to be done. You just need to be aware of it. Take a look here, which shows how he new system works

Is there a way to remove this warning? If there's nothing to be done, I would like to prevent it from making the preparation to v5 be yellow instead of green.

There isnt, but this is not stopping you doing anything in terms of upgrading

What happens to the current tags during an upgrade? Tags have been a mess for years, so I don't mind the change, but I'd like to know if I'm going to have to go back and re-tag every article.

I don't know if ALL the existing tags make it over but at least 14 pages of them did in my upgrade.

I am laboriously reviewing and deleting them one by one to get down to a sensible and curated set. If there's a bulk method to deleting them in batches I'd be thrilled.

A bulk method is coming in 5.0.4, it's currently in review.