$ scp host:/path/to/file /dev/stdout | \
    aws s3 cp - s3://bucket/path/to/file

This recently stopped working after upgrading:

ftruncate "/dev/stdout": Invalid argument
Couldn't write to "/dev/stdout": Illegal seek

I think I figured out why this is happening:

• New versions of scp use the SFTP protocol instead of the SCP protocol. [1]

• SFTP may not download sequentially

With scp I can give the -O flag:

Use the legacy SCP protocol for file transfers instead of the SFTP protocol. Forcing the use of the SCP protocol may be necessary for servers that do not implement SFTP, for backwards-compatibility for particular filename wildcard patterns and for expanding paths with a '~' prefix for older SFTP servers.

This does work, but it doesn't seem ideal: probably servers will drop support for the SCP protocol at some point? I've filed a bug with OpenSSH.

[1] "man scp" gives me: "Since OpenSSH 8.8 (8.7 in Red Hat/Fedora builds), scp has used the SFTP protocol for transfers by default."