Your 2015 Mac is already unsupported by Apple both with the hardware support/repairs and OS updates since a 2015 Mac is now considered "Obsolete" by Apple.
You can certainly keep using it until the apps you use can no longer do what you need them to do. At some point your access to online services will slowly stop working, but any offline parts of an app should still function until the files you need to access change their data format where the older versions of the apps are unable to recognize those data format changes. Every app and service will be different.
Usually people who depend upon a device for work (especially anything critical) should consider purchasing a newer device and demote the older device for a temporary spare device or to repurpose the older device to other things more appropriate for it.
FYI, when you do finally retire it or give it away or sell it, make sure you follow the instructions in the following Apple article so that you disassociate your accounts from the device & wipe your personal data from it:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support