What happens if I charge iPhone 11 with iPhone 7 adapter?

Nothing
The Obv differs is 5w vs 20w fast charging
And the connection port on the adapter, USB-A (5W) vs USB-C (20w)

@sardasa so you purchased iPhone11 finally? from Reliancedigital using Amex offer? How much effectively it costed to you?

I got the same charger as 7 for my xr so it wont make a difference other than faster charging i think.

No issue at all. In fact, they used to bundle in the same 5w charger back then.

it becomes iPhone 18

No issues, the wattage does matter for faster charging thats all. Yours one is probably 5w and it might take a bit longer to charge your iPhone. This should not be an issue for you.
Better buy some Qi or PD charger from amazon latter on.

I do not think there is a need to buy a new charging brick/adapter if you already have old one with you..
As already few people said many say that Lower Wattage will make the battery run longer (Low wattage keeps the battery cool which makes small difference in longevity) This is what many say..
So 5W or 10W or 18W all can work on your iPhone..
Infact Apple made a presentation to use ur old adapter and said they are saving environment..
Use old adapter if it works for u (slow charging) or else use some other android adpater(which is working in good condition)
Or else if you have macbook adapter u can use that (macbook and iPhones, Pixels, and few other electronics are using USB PD for fast charging) so they can work..

Slowly but steadily it starts turning into a iPhone 7, first internals then display and finally the entire body.

Like everyone said, no problem at all. Fast charging does kill the battery faster. But 5W charging is a bit too slow to be honest.