Depending on whether the respective kernel supported it, you were still unofficially able to switch removable SD cards into mass storage mode (though only with a rooted phone), although somehow, even if I remembered to officially unmount the SD card from Android first, it somehow still often led to mild filesystem corruption (luckily never anything fatal, though) that required regular chkdsk-usage.
> Or they could have figured out a new version of MTP that supports basic features like concurrent access and normal metadata. Or they could have gone for SMB/NFS over a virtual network link.
My current phone no longer supports the above mass storage mode-hack for the removable SD card, which annoyed me enough that I actually wrote my own SMB server app (https://github.com/buttercookie42/SimbaDroid), because all other SMB servers for Android that I'm aware of were either outright broken, unsupported, buggy or fiddly to use. Sadly the only open source Java-based SMB server only supports SMBv1, so you're stuck with that, and you still need root for full comfort, but within those limitations it works quite nicely.
I know that read/write conflict concerns are what got USB Mass Storage mode removed from Android, but surely there's some way to resolve that. Like it wouldn't bother me a bit if Android just locked the device and put it in "file transfer mode" when it's mounted on a computer, similar to how iPods used to and how Kobo e-readers do now. It'd be worth the universal robust multi-platform support.