Why do my Macbook Pro's batteries keep swelling?

Batteries deteriorate with age, as well as deep discharge and exposure to high temperatures.

Mac battery recommendations have moved on, because Apple systems have changed since your 2015 system was released. Now the T2 chip or Apple-silicon chipset monitors actual usage and can make adjustment to reduce time spent at 100 percent charged, which was found to reduce overall battery lifetimes.

Previous recommendations (no longer available online) included "exercising" your battery occasionally (thought to be once or twice a month) to reduce time spent at full charge. for your model, that had to be done manually.

Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a feature called Battery Health Management. Now, based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop well before 99 percent. 

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

In Intel Macs with T2 chip or Apple-silicon, Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.

NOTE that charging to 80 percent is a side-effect, NOT the GOAL.

The goal is to have some hurly-burly (percolating activity) around using your battery, either by your actually using it, or by leaving it less than fully charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower (thought to be around twice monthly) if you do not do so by your ordinary use.

There was also a production problem with certain units of the MacBook Pro 15-in 2015 model's battery, that inspired a service program (now expired) for battery replacement as a safety issue.

My MacBook Pro 2105 battery was replaced under that program. Later in its life, the replacement battery developed battery swelling as well.

This is far too small a sample to draw conclusions from, but note that Apple changed to actively monitoring battery usage and implemented Battery Health Management shortly after that 2015 model-year's troubles.

Punchline:

A newer Mac will mange "giving it some exercise' automatically., so adjustments to your usage patterns will NOT be needed.

With 20/20 hindsight, manually exercising that older Mac's battery may have prolonged its life, but that particular model's battery may just have been doomed.

There are environmental limitations for that Mac, both for storage and while in use, so a recommendation depends on exactly how hot / cold those extremes may be.

MacBook Pro (Retina, 15-inch, Mid 2015) - Technical Specifications - Apple Support

"Why do my Macbook Pro's batteries keep swelling?: Thank you so much for all of this information, in this post and your previous one. It's really helpful. So essentially, when I upgrade to a newer model (I'm looking at the 16" M4 Pro at the moment), I hopefully won't have to worry about battery issues as much, since the laptop will have a feature to help manage the battery and retain its health, even if I keep it plugged in all the time?"

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Purchasing AppleCare:

For future avoidance, maybe get AppleCare with this Mac, if not done do already. It would only last so long, but Apple does offer annual AppleCare coverage, after it ends, per their discretion. Go here, to see if it is an option for you: Check Your Service and Support Coverage - Apple Support