I read the following conclusion in an article about : "Online and Offline: To Hyphenate or Not"
(Author: Sheila Sanders )
Quote: "The rule, of course, is consistency: use the same form throughout a document. The normal progression in English is to move from two separate words, to a hyphenated word, to a single term. Thus, online and offline are no exceptions; they are simply moving through these stages very quickly. (...) "
In other words, if you write "off-line" then also "on-line",
... no idea if you have to proceed this also with "in-line" and "out-line" . 😄
A small note on e-mail: here "e" is the abbreviation for "electronic" and I see the hyphen as helpful - for me a difference to off-line.
E-Mail without the hyphen could occasionally be confusing in other languages, since there exist a word "email",
but with the meaning of "Vitreous enamel".
In the Wikipedia article there is therefore a remark: "This article deals with the enamel coating of Email. For electronic mail, see E-Mail." (translated)