Are you talking about the Handheld Sync functionality?
Because Handheld Sync still uses a Backslash when creating Playlist files to put on the target device, whether that is a directory on a hard disk in a PC, an SD card mounted on a PC, or an Android device connected to the PC.
My Sony Experia XZ Premium is still on Android 9, so it handles the Backslash and the Forward Slash without any problems, but I hear that Android 10 can't handle Backslashes. This has been reported on the forum several times.
I tested with the "M3U (relative paths)" format of Playlists.
PS: If anyone is going to do some work on Handheld Sync, please run a few Syncs in a row and observe what happens to the [Name] field and the listed metadata on rechecking Sync, and for the second and subsequent syncs. Run "Recheck Sync" multiple times in succession. It is still broken, at least for me, and always sends the full list of files each time, as it can't match what is on the device compared to what is in the Sync list. It also loses the Cover Art display for any file listed as "On Device", even if it was only just Synced, and the "In Queue" version with the correct [Name] has Cover Art. This stuff has been reported multiple times by others.
I would really like to use Handheld Sync more, but...