Kermit is the name of a file-transfer and -management protocol and a suite of computer programs for many types of computers that implements that protocol as well as other communication functions ranging from terminal emulation to automation of communications tasks through a high-level cross-platform scripting language. The software is transport-independent, operating over TCP/IP connections in traditional clear-text mode or secured by SSH, SSL/TLS, or Kerberos IV or V, as well as over serial-port connections, modems, and other communication methods (X.25, DECnet, various LAN protocols such as NETBIOS and LAT, parallel ports, etc, on particular platforms).

This software will help you transfer software to your classic Macintosh (e.g. MacOS System 6, 7, 68k Macs, etc) using the serial port. Very simply, this software runs on a modern host computer, and behaves like a BBS would back in the day. You connect your Macintosh to the host computer using serial, and then, using your vintage computer, you can search and download files from online archives (e.g. Macintosh Garden, Mac Repository). However, this "BBS" has only one user, you!

I don't see why you couldn't use this pseudo-BBS to transfer software to other kinds of computers as well.

UI-less tool to connect Android phone with desktop. Copy files to and from your mobile. For use with the Device Connect application from the F-Droid appstore (https://www.f-droid.org/en/packages/net.dcnnt/).

This package ports the XMODEM, YMODEM and ZMODEM protocols to Python. We try to implement the protocols as minimalistic as possible without breaking the protocol specifications. All modem implementations must be given a getc callback to retrieve character data from the remote end and a putc callback to send character data.

Application for transferring files to mobile devices. Throws up a QRcode in a terminal and spawns a web server. Scan the code, download the file.