I get the following:

zira:~> /usr/bin/ssh -vvv gcc202.fsffrance.org
OpenSSH_8.4p1 Debian-2, OpenSSL 1.1.1h  22 Sep 2020
[...]
debug1: Authentication succeeded (publickey).
Authenticated to gcc202.fsffrance.org ([195.170.63.224]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug3: send packet: type 90
debug1: Requesting no-more-sessions@openssh.com
debug3: send packet: type 80
debug1: Entering interactive session.
debug1: pledge: network

and at this point, ssh is stuck. This is probably a server side issue,
but on the client side, I want to interrupt it. Neither Ctrl-C, Ctrl-\
nor [Return]~. works.

Here's the backtrace:

0x00007f65de8097c6 in __GI___select (nfds=7, readfds=0x55d3fbfb6f90,
    writefds=0x55d3fbfc4100, exceptfds=0x0, exceptfds@entry=0x7ffcb7d05ed0,
    timeout=timeout@entry=0x7ffcb7d05ed0)
    at ../sysdeps/unix/sysv/linux/select.c:41
41      ../sysdeps/unix/sysv/linux/select.c: No such file or directory.
(gdb) bt
#0  0x00007f65de8097c6 in __GI___select (nfds=7, readfds=0x55d3fbfb6f90,
    writefds=0x55d3fbfc4100, exceptfds=0x0, exceptfds@entry=0x7ffcb7d05ed0,
    timeout=timeout@entry=0x7ffcb7d05ed0)
    at ../sysdeps/unix/sysv/linux/select.c:41
#1  0x000055d3fbe1b1e7 in client_wait_until_can_do_something (
    rekeying=<optimized out>, nallocp=0x7ffcb7d05eb4, maxfdp=0x7ffcb7d05eb0,
    writesetp=0x7ffcb7d05ec0, readsetp=0x7ffcb7d05eb8, ssh=0x55d3fbfb7590)
    at ../../clientloop.c:562
#2  client_loop (ssh=ssh@entry=0x55d3fbfb7590, have_pty=1,
    escape_char_arg=<optimized out>, ssh2_chan_id=ssh2_chan_id@entry=0)
    at ../../clientloop.c:1368
#3  0x000055d3fbe0dd4d in ssh_session2 (pw=<optimized out>, ssh=0x55d3fbfb7590)
    at ../../ssh.c:2192
#4  main (ac=<optimized out>, av=<optimized out>) at ../../ssh.c:1687
quit)