The Librem 13 version 1 (l13v1) supports an external microphone via that 3.5 mm audio jack. @mladen told me this and I just verified this with a TRRS (connector has tip, 2 rings, and sleeve) microphone. @mladen told me the CTIA standard is followed for the assignment of roles to the 4 contacts.
I don’t know when this functionality was dropped, but it appears to be gone by the l13v3. I don’t know whether the l15 ever had it.