That doesn’t make the source code proprietary or non-open, it just means it isn’t a community driven project.

Slackware is still around, no past tense. What makes you think it was closed source?

Also, interplanetary timezones and leaps.

since it’s JIT, it’s actually faster than Python and Java in most cases.

Java is JIT’d too, and Python can be depending on which runtime you deploy.

What has changed about RPN or calculators in the past year?

You can check the release notes to be sure, but generally you can just perform the update and move on with life. Backing up your data is always a smart precaution.