Im not familiar with the Asterisk integration ilself, but I'm generally familiar with HA and community addons to ask if this isn't that big a deal? If I'm interpreting the pull correctly, HA is dropping the native integration for a mailbox feature that only Asterisk uses, which technology itself is outdated. Looks to me like the Asterisk implementation itself is a HACS resource, and not part of the HA mainline code.

Could be the case the Asterisk community project maintainers will either update their code to live without the native feature, or adopt the deprecated code to maintain it as part of the community add-on.

The corpo-speak seems pretty straightforward to me, though: the asterisk-mbox integration code is unmaintained, outdated, or has fallen out of compliance with the prevailing API usage so it's planned for being removed from mainline. Happens frequently with legacy code in large projects.

I wouldn't bring out the pitchforks just yet, OP. Good chance the Asterisk repos will update with a fix or a workaround in the coming weeks to months