If you use Microsoft's Outlook email client with any regularity, you may well have noticed the "Try the new Outlook" toggle appear at the top of the app in recent months. Maybe you've even tried it. Either way, the slow demise of the "classic" version of the app has now been outlined.
The New Outlook is currently in what Microsoft is calling "Preview" mode. This means that although they're confident it should work fine for many users, it is not feature complete, and some people will find that various integrations and / or specific use cases will not work as expected
Although the exact date hasn't been nailed down, Microsoft is currently planning to remove the app from Preview status next year. From this point, this new version will become the default when you go to install Outlook. About a year after that, the option to opt out of using the new version will be removed. Microsoft is currently planning on removing all support from the "Classic" version in about five years, although the point it's effectively impossible to install may well come before that.
While this probably won't be an issue for most, as noted above, there may be problems with certain 3rd party integrations. The way the new app works is fundamentally different to the old one, so these integrations are unlikely to "just work" when switching, and it is very likely that the authors will need to rewrite them.
It may well be worth ensuring you've at least tried the new version well before the eventual cutoff. If it already does everything you need, then great. If not, you'll need to identify what's missing. If it something Microsoft is planning to fix in the next few months (which includes Offline Support, "msg" and "eml" file support, and "pst" file support), then it'll just be a matter of waiting. However, if it's an issue with 3rd party software, it will be worth chatting to that company to find out what their migration plans are if you still have a relationship with them. If it's something historic that you no longer have ongoing support for, you will need to start making alternative plans for whatever that integration is.