Has the countdown for iPhone 6s begun in 2019?

Launched in 2015 the iPhone 6s is the name for Apple's 2015 line-up, consisting of iPhone 6s and iPhone 6s Plus. Apple did not make any changes as far as design was concerned but for the first time introduced 3d touch, which was quoted to be a innovative feature. However, that was back in 2015 now mobile phones have come a long way from that technology. Mobile phones have upgraded and are much more advanced. Although unlike iPhone 6, iPhone 6s can now benefit from the new IOS 13 it's just a matter of time when iPhone stops iPhone 6s to benefit from newer iPhone updates.

Sell my mobile phone if it's out of warranty?

Yes, you can, for instance, if you have an iPhone 6s for sale, you can sell it without any hesitation but soon maybe within a year, it will be a lot less desirable. The same will be the fate of iPhone 6s; currently, it has missed the cut and iOS 13 can be used on it but soon the newer iOS 14 won't and that will burry iPhone 6s just like all its other predecessors. Used iPhone 6s can be said to have almost 2 years left on the clock and after that iPhone 6s in used condition wouldn't worth anything. Within two years or so iPhone 6s refurbished and even in mint condition wouldn't be able to attract any price. Hence it can be said that you should sell mobile phones before they reach that state else you will be left with only one option to recycle mobile phone for free.

Can I get cash for mobile if Apple's new IOS is not compatible with my old iPhone 6s?

where can I sell my phone and buy iPhone x?

You can still buy used iPhone 6s if you are on a tight budget and want to save up.

However, you should always try to sell mobile phones before the latest update is not available for your model. You can always sell mobile phone online even after Apple has stopped the latest IOS upgrade on your model.
You should trade in a mobile phone before it reaches a state where Apple can't let new IOS being installed in it. This has been happening for years, and whenever any mobile phone is left-back, it just becomes the most undesirable mobile phone and starts vanishing from the market. For instance, we do not see any iPhone 3 with anyone anymore. Moreover, slowly a point comes when no one is willing to offer money for mobiles of that age. Although many mobile phone recycling companies advertise "old phones for cash" they lose the interest in buying the models and removes these models from the list of mobiles for cash. That is one of the reasons why you can't sell the iPhone 5 in 2019 although it works as a mobile phone but still does not allow you to enjoy the latest technological advancements Apple has to offer. In this day and age, mobile selling is much easier than the past, and you should consider selling your old mobile phone the moment you don't need it or have upgraded to a better version. People who take mobile phones as other items which hold their price lose big time in the end. For example, if you happen to own an iPhone 4, you can sell the old iPhone 4 in 2019. It's unusable.

Shall I buy a refurbished iPhone which has no support from Apple, and when shall I sell my device?

Yes, as long as you know it will let you install the latest IOS on it else, you should reconsider buying it. The sooner, the better, the latest mobile phones such as iPhone x in used condition will hold its price for a while but soon when newer iPhone is released, and iPhone X gets a bit old it will start losing its charm and value. Old iPhone loses almost 3% of its value every month after the launch. So almost in 36 months, the mobile phone becomes not desirable. From experience, it can be advised that you should not buy used iPhone 6 in 2019 due to compatibility issue with the new IOS 13.

Further Readings:

Rise and fall of iPhone 6

Why is the LCD of Samsung phones so expensive to replace

10 Creative Ways to Use Old Smartphones