Microsoft started rolling out changes to Outlook.com back in March
https://www.theverge.com/2018/3/14/17121032/microsoft-outlook-web-redesign-features
but, at that time, some new features were still optional. As with the way that Microsoft always seems to work though, features which are initially optional eventually get forced onto everyone, so it seems that they must have reached that stage now.
Microsoft's development teams are staffed by people who know that they need to keep 'improving' things in order to keep their jobs. As a result,users of Microsoft products keep getting lots of new (but generally unwanted) features foisted upon them.
I wouldn't touch Hotmail/Outlook with an infinite number of the proverbial bargepoles. If you don't like the changes I suggest switching to a more user-friendly service, such as GMX. (You needn't lose your Hotmail address. You can configure the 'mail collector' facility to automatically forward mail sent to your Hotmail address to your new one).
https://www.gmx.co.uk/
Alternatively (or as well) you might like to consider ditching webmail altogether and accessing your email accounts (whether they be with Hotmail, GMX or anyone else) through a proper email client. Then you won't have to concern yourself with the ever-changing interfaces of web-based services:
https://www.thunderbird.net/en-GB/