Incurs_seek_penalty column of sys.dm_os_volume_stats
This is now officially documented, with the following values:

Indicates the type of storage supporting this volume. Possible values are:
0: No seek penalty on this volume, typically when the storage device
is PMM or SSD
1: Seek penalty on this volume, typically when the storage device is
HDD
2: The storage type can't be determined when the volume is on a UNC
path or mounted shares
NULL: The storage type can't be determined on Linux operating system
Applies to: SQL Server (starting with SQL Server 2019 (15.x))

It looks like this is related to the low level DEVICE_SEEK_PENALTY_DESCRIPTOR structure (thanks, LowlyDBA!) - possibly in an attempt to check if a drive is an SSD or not (that approach is also discussed on Stack Overflow).
It would still be interesting to know if this information about the storage is (or will be) used by SQL Server (e.g., for updated I/O cost estimates, or performance optimizations inside the storage engine).