Modern Thinkpad notebooks (like my T14 Gen 2) are initially configured to use a not-quite-sleep power state S0ix aka Modern Standby in lieu of traditional ACPI S3 sleep. Since this does not currently provide the touted benefits (e.g. a notably faster resume-from-suspend experience and comparable power draw while sleeping) it's preferable to disable it.

This is done via a BIOS config option:¹

That's all that's needed to get a normal system sleep experience back (no more fans continuing to run after the lid is closed). However, on some systems this will expose an ELAN touchpad firmware bug which causes an interrupt storm (noticable via the system fans constantly running loud) after resuming from sleep. Easiest way to apply the update is to boot into a Windows partition and run Lenovo System Update -- the firmware update will be listed among the "critical" updates to apply.

1. This option may not be available without first updating the system firmware ↩