Water resistant is a lower threshold than waterproof, the latter being harder.

Many phones are water resistant with manufacturers guidance as to time and depth of water it can survive, eg iPhones https://support.apple.com/en-us/108039

As far as I’m aware, after you go to Apple or Samsung for a official battery replacement, there is no advisory issued to say the repair affected the previous level of water resistance

We've had devices like this forever.

A: add a gasket.

B: glueseal everything except for the battery, connected via exposed gold pads.

The current issue with battery replacement is mainly the fact that they're glued to the phone. Which make them very dangerous to replace, as newbie might try to pry off the battery with metal tools (and you don't want to know what happens when a lithium battery is pierced.)

The only thing that will change as far as I know is that battery won't be glued anymore. You'll still have to open the phone with screw like you did before.

Yes, many manufacturers know. I have a number of Motorola Defy phones from 2010/2011 with replaceable batteries, all waterproof. My daughter left hers in her back pocket when she put her trousers in the washing, the thing went through a whole washing + spin cycle and still worked.

I.e. Samsung Galaxy S5 was IP67, Samsung Galaxy Xcover 6 Pro was IP68.