My '16 Outback has a phantom electrical drain (diagnosed as the thematics module) - if the car sits for any time beyond overnight, I return to a dead battery. And the wait for the replacement part is estimated to be until July! I'm writing this in March.

So, I have been thinking about installing a battery cutoff switch so I can disconnect the battery when the car sits.

Recommended or not? What are the negative outcomes of doing so?

There is a Subaru battery class action lawsuit. See Home | In re Subaru Battery Drain Products Liability Litigation

 

Since it appears you need a temporary solution, how 'bout a temporary fix? Um, just disconnect the battery ground terminal. And don't close the hood all of the way so you don't accidentally lock yourself out.
Really, it only takes a second, and a 10mm wrench to do this.
You will lose all of your customization settings, but you will anytime you disconnect the battery . . .

 

Um, just disconnect the battery ground terminal.

I would recommend AGAINST this. Most newer vehicles have a voltage and temperature sensor on the Ground terminal. There are several TSB's out there with very specific instructions on how to deal with this, which side to remove, and what torque to use (very low!) when reconnecting. Break the sensor, and you'll be out about $100 and possibly deal with a new cause for your dead battery while you try to figure out why your alternator is no longer charging properly.

 

Huh. Well, that's what it sez allright; my bad. Sorry.
Every service manual I have ever looked at starts off with "disconnect the battery", generally understood to mean disconnect the negative terminal. Don't have a manual for the '22 car, and short of replacing the battery if/when it becomes necessary, I do not plan on working on it. Gold+ warranty, and it will be gone before that runs out. Probably long before .
The '16 forry doesn't have that sensor, nor does my '10 Tundra; just a regular ol' clamp.

 

Apology accepted!

My '18 Forester (SJ model like your '16) has an assembly on BOTH battery terminals. The negative pole has a current and temperature sensor, but the design is even more convoluted that this on, with an angled post clamp. There is another TSB specific to that design. The positive pole is actually a remote fuse block with 2 or 3 wires attached, going to the alternator, starter and underhood fuse housing.

 

This is one of the older TSB for the 2015-2020 battery sensor terminal post. They seem easy to break when removing, so I'd proceed with caution when doing so. Applicable to my 2018 (SJ model Forester) along with Outback & others.

 

I wonder what will happen when I disconnect that sensor and replace the ground cable/battery clamp assembly with a regular one? I'll report back . . .
PS: No sensors on our 2016 Forester battery. The '22 car has the sensor on the ground terminal, and a large assembly on the positive terminal with three or four cables attached to it via ring-terminals.
Replacing the battery in my MacbookPro was trivial compared to dealing with the one in this new Outback. "Thanks for that, Subaru."

 

I wonder what will happen when I disconnect that sensor and replace the ground cable/battery clamp assembly with a regular one? I'll report back . . .

There have been a few other threads about the Smart Charging system on the various Subaru boards. The Factory Service Manuals also have diagnostic descriptions of what happens. Since 2015 there have been a few different implementations and a different connector on the top of the alternator that receive signals from the ECU that in turn determine the charging needs based on these battery sensors. Ditching the sensor gets you one of several responses, depending on the model or year. They range from DTC's that impact driveability, dash lights, undercharged dead batteries, overcharged and cooked batteries, or seemingly nothing at all. So go have fun and report back which of the above you end up with!

 

There is a listing in the Service Manual for all the inputs to A/S-S, and battery voltage is certainly one of them. That the absense of a valid value disables the system entirely goes beyond just a minimum voltage threshold for whether to halt the engine and for how long. Interesting find.