> when they decided to dictate which Linux filesystems I may use

That’s an odd framing. How about “when they didn’t support the filesystem I use”?

If some software is only available on Windows I doubt anyone would say that the vendor is trying to “dictate what OS I use”.

The framing is not odd at all. Your framing makes it sound as if this was some requirement that had been known since the beginning rather than the typical "Good news! For our own corporate reasons we have decided to make your life better by jumping to the top of your todo list and breaking your things!" Thanks, I hate it.

To be clear it is trivial to thwart the check, but shared library shims can't fix the real problem which is pointless corporate contempt. Particularly if I'm paying for it.

Like everyone else, they have to decide what they support – their core product pretty heavily depends on known file system semantics – and they gave advanced notice specifically so you didn’t have to “drop everything” if you for some reason cannot have a partition using a supported file system.

Put another way, do you think the combined users of file systems which aren’t supported ext4, xfs, btrfs, or zfs are willing to pay more or would quietly accept the possibility of data loss? I doubt the former is true and have absolute certainty that if there was a bug using an unsupported file system that would result in angry, hyperbolic blog posts saying Dropbox is unsafe and will lose your data.

Oh are those all supported now? They've clearly backtracked massively. When I bounced they were insisting on ext4 only. Glad I didn't bother rebuild all my machines to ext4 only for them to change their damn minds. They gave us less than three months notice. I'm not playing chicken for three months paying their professional tier in the hopes they change their mind. Freaking circus.