The current code for session support determination looks at servers instead of topology description, which is because topology description didn't have the necessary fields implemented until 1516. But now it is possible to implement session support logic mirroring the text spec.

Interestingly, although the spec claims that standalone deployments do not support sessions, both 3.6 and 4.0 standalones in Evergreen do in fact advertise logical session timeout in ismaster.