Spent some time investigating this and noticed the difference to master is that the file-open handler is not stored in wmWindow.modalhandlers but in wmWindow.handlers, which have a much lower priority. So probably some other handler returned WM_HANDLER_BREAK, blocking the handling of wmWindow.handlers (just guessing on this, didn't investigate deeper).

Also reopening as design task. Like I said, this change seems to be crucial to some people.

Even though the workflow people used with the old behavior may abuse the original idea of the feature, I'd say we should keep supporting it. Maybe not many people use "Lock to Cursor" at all, but apparently for those who do, this change is crucial. BTW, I don't remember hearing anybody complain about the old behavior.

Yes it can. Draw-manager goes into same direction as I intended to go with this, so it's totally redundant now.

Really can't recreate this here. The first thing to try would be using factory settings and changing to Maya preset afterwards.

Committed rB0dd6e5bfee3de. Thanks all!

Requesting changes since we apparently agree that this isn't the best solution.

I also don't think we need to touch 2.7x code here. Forward compatibility should indeed be solvable by writing data that pre-2.8 Blender can read.

Committed rB46fc0bb87ebda, thanks a lot for the review!

So quick update...
All review points should be addressed. The diff is a bit smaller now ;)
After latest patch update, custom transform orientations may behave a bit glitchy but that is kind of intentionally. D2687 should fix these by moving transform orientation from scene to workspace level. It should be applied right after this.