Ssh – MTU on server accidentally set to 0, can’t ssh into it

You are not using scponly here. You have restricted the user upload to using SFTP, so you can't use scp on that account since scp relies on an interactive shell or at least a shell which supports certain commands (like scponly or rssh).

Use sftp or another SFTP client (e. g. lftp) to access that account or remove the ForceCommand and ChrootDirectory directives from your sshd_config.

The client can perform Path MTU Discovery, which is the smallest MTU on the complete path between two devices, and then reduce the MTU to match. This is particularly necessary for traffic that shouldn't be fragmented, such as IPSec.

As Shane Madden mentions in the comments above dropped ICMP packets in the path will break this. I suspect there are some switches on the market that have all ICMP blocked by default, so I would check to see whether any new switches or routers were introduced and whether they need to have ICMP traffic enabled.