how long will iPhone 5s be supported

Typically, Apple's policy on their iPhones is that they will be supported up to 5 years after a particular model is no longer in production. There are some exceptions due to where it is required by law to extent that period.

Since the iPhone 5s went out of production in March 2016, your iPhone should still be supported until 2021.

Ref: Vintage and obsolete products - Apple Support

Only Apple would know. We're only users here.

You're welcome.