First, we're volunteers here. If you want a formal, official response, you need to go elsewhere. However, there is an RFC in place for it (https://www.rfc-editor.org/rfc/rfc8792.pdf ), it's been a common line length for decades, many people prefer it for it's tighter construction, and there's even an addon to enforce it. So, just looking from the outside, I would guess that the option is there as a user convenience. And, even if it's a poor explanation for you, it's still there. I should also add that it's an option in other email clients, such as TheBat and Becky!, and possibly others. Just my two cents...

Thank you for your time and patience sir. Sorry I was a bit mad :< Is a decent explanation. I'm not sure what to believe, I don't know world-wide but Google mail is really popular around me and, to make emails send from Thunderbird to google mail format weird (even if Google are in the wrong) is an odd decision.

The line length is only used for plain text emails. So basically it is not used at all.

The issue is Thunderbird under the hood converts HTML mail to plain text if there is no formatting. That is the problem and when I submitted a bug requesting a change of default the bug was summarily closed. Apparently the folk in the development team are old UNIX users and they like plain text.