I don't like this advice, because most people do not know how to create and use these things safely and securely, i.e. created on a clean and airgapped computer, and only ever recovered on a clean and airgapped computer. Most people would create these back ups by writing their seed phrase in a text file, copying it to their external storage, and then deleting the text file, all on their usual every day computer while connected to the internet. I'm sure I don't need to point out to you all the things which could go wrong with such a method, but many newbies will be unaware of the risks involved. Conversely, everyone is able to write down some words on a piece of paper safely and securely. Redundancy is mandatory as I mentioned above, but you can achieve redundancy simply by making more than one paper back up and storing them in separate locations.

Don't do this. Don't store anything valuable in an email account, ever. I'm not sure what you mean by storing it in an email account that isn't online, but if you are storing something valuable on an airgapped computer then you should encrypt it, not put it in some email software.