Lemmy uses Markdown for storing/displaying comments, specifically the CommonMark flavor with some extensions, as documented here: https://join-lemmy.org/docs/users/02-media.html

And yeah, no, that doesn't have a way to underline,

except

maybe

one

of

these

here,

depending on the Markdown implementation in the client. Some clients might also choose to underline the highlighting, that's normally 𝘪𝘵𝘢𝘭𝘪𝘤.

Links may be underlined as well.

Well, and then you've also got this thing, which is certainly a line under this text:

On Lemmy, we also got strikethrough. Priorities, I guess. 🙃

But yeah, no proper, consistent underlining in Markdown/Lemmy. 🥴