WordPress editor: “pick from existing tags” is gone?

You mean this?

It’s been on discussion 2 years ago and I think it is one of the reasons why Gutenberg is not really beloved by many people.

If you don’t rely on Gutenberg and need the feature so badly, you can use a plugin that enables the Classic Editor for posts, at least.

I know that is not a perfect solution, but I’m not aware of any plugin to do what you need for Gutenberg and maybe your posts are of simpler nature, so that they won’t need Gutenberg at all.