Typically fast charging reduces backup time and life. Now a days almost all handsets are coming up with different fast charging capabilities. I'm currently using Galaxy Note9 which has fast charging features as well. When I connect the supplied charger it starts charging with fast charging always. Fast charging is really helpful. But when I charge my phone at night while sleeping, it also uses fast charging. My question is during night time, fast charging is not required and normal charging is beneficial. So, why the manufacturer doesn't keep normal charging mode so that it can be used at night time for getting better backup time and better battery life.
I've seen this functionality in a Dell XPS laptop BIOS, a sort of a calendar where you can set days/times where fast or normal charging is used. Not for phones though. The closest you can get is to have two chargers - one capable of delivering maximum current and another "dumb" 100-500 mA, and use the latter one at night. I started doing this as soon as I realized that charging can't be tuned in the phone. The results are promising so far - my phone is on the same battery for 3+ years going strong, while the same age and model operated by my SO is at the end of the second one :-).
The main reason why fast charging is not advisable to be used always is due to the heat generated by supplying higher than usual current that the battery normally experiences. If the phone has a good enough heat dissipation capacity from the battery to the outside world, it's normally okay to use fast charging always.
Also, if you keep the battery temperature low, fast charging will not reduce battery capacity. Charge-discharge cycles will.