Can we still get iPhone 6 battery replacement for $29?

Yes * Through December 31, 2018, the out-of-warranty battery service fee is $29 for all eligible iPhone 6 or later models. Battery service at $29 may be limited to one repair per iPhone.

Make a genius bar appointment with Apple for an evaluation

That's the price in the United States directly from Apple for all models starting from the iPhone 6. Going to an Apple Authorized Service Provider may come with a different price. Many are matching or at least close to Apple's price.

They may not do it depending on the condition of the iPhone. If the screen is cracked, they will probably only do it if the customer agrees to pay for a screen repair first.