Ooooh... sounds like a fun challenge :D I've typed this out straight into the HN input box so it almost certainly doesn't work and will be full of typos and escaping bugs, but for illustrative purposes: here's what a ~8 line Matrix client for basic bi-directional chat in a given room could look like (complete with login) in almost-bash, with deps only on curl and jq:

    USERNAME='@whoever:matrix.org'; SERVER='https://matrix.org'; ROOM='#test:matrix.org'
    read -s -p "Password for $USERNAME:" PASSWORD
    TOKEN=`curl -X POST $SERVER/_matrix/client/r0/login --data "{ 'type': 'm.login.password', 'login': '$USERNAME', 'password': '$PASSWORD' }" | jq .access_token`
    ROOM_ID=`curl $SERVER/_matrix/client/r0/directory/room/$ROOM | jq .room_id`; curl "$SERVER/_matrix/client/r0/join/$ROOM_ID?access_token=$TOKEN"
    (while true; do SYNC=`curl -s $SERVER/_matrix/client/r0/sync?access_token=$TOKEN&timeout=30000&since=$SINCE`
     echo $SYNC | jq ".rooms.join.$ROOM_ID.timeline"
     SINCE=`echo $SYNC | jq .next_batch`; done) &
    while true; do read -p "<$USERNAME> " INPUT; `curl -s -X POST $SERVER/_matrix/client/r0/rooms/$ROOM_ID/m.room.message?access_token=$TOKEN --data "{ 'body': '$INPUT', 'msgtype': 'm.text'}"`; done

Spread out a bit more with comments:

    # set your matrix ID & server URL, and the room you want to chat in:
    USERNAME='@whoever:matrix.org'
    SERVER='https://matrix.org'
    ROOM='#test:matrix.org'

    # prompt for a password; log in and grab an access_token
    read -s -p "Password for $USERNAME:" PASSWORD
    TOKEN=`curl -X POST $SERVER/_matrix/client/r0/login --data "{ 'type': 'm.login.password', 'login': '$USERNAME', 'password': '$PASSWORD' }" | jq .access_token`

    # resolve the room alias (#test:matrix.org) to a room ID (!vfFxDRtZSSdspfTSEr:matrix.org)
    ROOM_ID=`curl $SERVER/_matrix/client/r0/directory/room/$ROOM | jq .room_id`

    # check that you're joined to the room (redundant if you know you're already there)
    curl "$SERVER/_matrix/client/r0/join/$ROOM_ID?access_token=$TOKEN"

    # set a background loop running to receive messages, and use jq to filter out the
    # messages for the room you care about from the sync response.  For now we print them
    # as JSON pretty-printed by jq, but that's not too bad.
    (while true;
        do SYNC=`curl -s $SERVER/_matrix/client/r0/sync?access_token=$TOKEN&timeout=30000&since=$SINCE`
        echo $SYNC | jq ".rooms.join.$ROOM_ID.timeline"
        SINCE=`echo $SYNC | jq .next_batch`
    done) &

    # set a foreground loop running to prompt for your own messages and send them
    # into the room as plaintext.
    while true;
        do read -p "<$USERNAME> " INPUT;
        `curl -s -X POST $SERVER/_matrix/client/r0/rooms/$ROOM_ID/m.room.message?access_token=$TOKEN --data "{ 'body': '$INPUT', 'msgtype': 'm.text'}"`
    done

If I have time I'll actually try running & debugging this to be usable and edit the post (but got to run into a meeting now :( ).

Needless to say, this'd be much prettier on Python - and you'd prolly want to use a nice SDK like https://github.com/poljar/matrix-nio (see https://matrix.org/blog/2019/07/03/usage-of-matrix-nio/) so you get things like E2E Encryption for free. But doing it in plain bash hopefully gives more of an idea.