Can I charge my iPhone 15 or 16e with an old MacBook Pro 87W adapter?

Yes. Your 87 Watt power adapter will charge like a 20W power adapter, because that is considered Fast Charging and is the fastest you can charge your iPhone. Charging with more wattage as you are doing will not charge any faster nor damage the battery. As Kurtosis12 correctly states, the iPhone only draws out the power it needs.

Yes, the phone will only draw as much power as it needs, you won't overcharge it.

Hello~ Yes, as pointed out by my other esteemed colleagues. I am in fact using my 87 watt adapter to charge my iPhone right now.

~Katana-San~