Using classic Outlook for Windows in business environments
Hi Tony_MC - You are not being kicked or forced to get a Microsoft 365 Subscription. If you would like to continue to stay with the same Outlook experience you have been using, you can upgrade to Office 2021 which does support Modern auth against Outlook.com sync. If you don't want to upgrade and just want a free offering, we do have the New Outlook which supports multi account, a free version of Mac Outlook, as well as Outlook for both iOS and Android.
As you mention there are other solutions like Thunderbird that do also support OAuth and can sync your outlook.com mailbox.
You can also continue to use your current setup until Basic Auth is fully deprecated, which we will ensure we give well and ample notice before that happens. However, if you are using that you might see increased authentication prompts as we mentioned due to the increased security settings that we have had to deploy to ensure we are protecting accounts from potential attack.