Version 1 import
Chen Lung is currently entering a long list of import problems for 1.x scores. There was a conscious decision to not be concerned about version 1 scores not properly importing into version 3. This was a logical decision. Few people will upgrade from version 1 to version 3 without first upgrading to version 2 so the number of people this will affect will be very few. Version 2 was designed to import version 1 scores as well as possible. Version 3 supports import of version 2 scores. If there is a problem importing a score from version 1, it should first be imported to version 2. If version 2 imported it correctly and there is still a problem, then this is a bug that version 3 needs to fix.
Comments
I haven't been involved in the software as I was a few years ago, and I've not been on Telegram either.
As for the jump from 1 to 3, not everyone will be aware/able/want parallel installations (though I personally do have a version from each series). Series 2 might(?) been downloaded more times than 1 (particularly on account of tablature), but people will still encounter scores in the latter - particularly entries on .com that haven't been updated for a long time, and possibly elsewhere, so I don't think we should completely abandon support if a solution is available - the four issues I posted are the only ones found so far, and that (also another by someone else) might be all.
For context, see this: #292394: Barline in 1.x score incorrectly presented
In reply to I haven't been involved in… by chen lung
I agree with your other post that we should wait for someone like Anatoly to make a decision if these issues should be addressed.
MuseScore 3 has direct MuseScore 1 import, a separate read114() function. So if there are issues with this, these should get addressed. Anatoly created #274863: [EPIC] Importing <= 1.14 version scores to 3.0 for this almost a year ago, I take that as a decision that none of those are by design
Another reason: series 2 is no longer being developed, which could present some issues:
Bugs with importing 1.x scores
Operating systems that did not or cannot support it in future
MuseScore 3 will probably have better remastering tools, too.