The guides you shared don't fully reflect the changes made to more recent versions of Electrum.
Since desktop version 4.6.2 and mobile version 4.7.0, all you need for making a transaction with a fee rate of less than 1 sat/vbyte is to be connected to a server that accepts such transactions.
In older mobile versions, it was not possible to make a sub 1 sat/vbyte transaction at all, and in older desktop versions, you needed to use a command in the console.
Thank you for detailed explanation.
By the way, I'd like to ask you one question about Replace By Fee. Sometimes, I can bump the fee rate of my pending transactions. Some other times, I can not bump the fee. I don't remember the exact message from Electrum but it's like the server does not allow me to bump fee, something like this.
Is it like Bitcoin nodes can customize their node settings and allow Bitcoin users to bump fee rate or don't allow it?