The change was introduced in 1e731869b2df ("Set the ruid to the euid at
startup as a workaround for a bug in pam_ssh.").

However, pam_ssh was fixed to have matching real and effective UIDs in
d90278190841 ("Complete rewrite of pam_ssh(8). The previous version was
becoming hard to maintain, and had security issues which would have
required a major rewrite to address anyway.")