And batteries can charge pretty fast these days. Modern pocket supercomputers keep track of battery temperature to keep things within defined limits during charging.
AFAIK the real problem, longevity-wise, for these batteries in normal use is the time spent at extremes of charge (<20% or >80%, ish).
That all said: Sure, some phones have options.
My Samsung phone does some man-behind-the-curtain tricks to attempt to make it reach 100% just before it predicts that I'll unplug it (eg, when I wake up). The idea is to maximize the charge on the phone while also keeping it at 100% SoC for as little time as practical. This probably works great for people with regular schedules (which is to say: people who are not like me).
This phone also lets me explicitly disable various fast-charge modes. I think there's at least two different modes that I can turn off (but I leave them all turned on).
And there's also a mode that limits the maximum charge to 85%, to promote long-term battery health. I have this mode engage automatically when using wireless charging, which is something I only do with the wireless charging cradle on my car's dashboard. (I do want the phone to be powered while I drive, but I don't normally need anything to work extra-hard to cram that last 15% into the battery when I'm on a long drive. It's a good balance, for me.)
More to your question the DIY solar community is torn between the benefits of float voltage which is a pretty low charging rate. I bought some cells (not for my house) and on the data sheet they were fine with .1C. They were more concerned with temperature and very high C rates hurting the lifespan. Since I was going to AC couple I was more worried about total system efficiency so similarly staying under a rate that heats the battery up and above the idle power of the inverter was my goal.
USB-PD now has a standard for having sink devices request a particular current from the charger, meaning that you could actually remove a converter from the sink device side, because with a programmable source, the current limiting happens on the power supply side. Cool stuff especially for small electronics like wearables.
Generally though if you want to charge with USB-PD and accept all kinds of chargers, the sink device will have to have its own charge control PMIC.
All this negotiation happens over a side band via the "CC" pins on the USB-C connector.
Devices that support it can potentially charge more efficiently (and by extension more quickly, if they're limited by heat dissipation), but they can't rely on any USB-PD adapter supporting it.
It's also different from AVS (which Apple's adapter seems to support), which allows controlling the voltage in finer steps, but not limiting the current in the same way that PPS does.
If a device charged overnight does not support limiting charging to 80%, a 5W charger means the device spends less time between 80% and 100% versus a higher wattage charger.
For a newer device that does support limited charging to 80%, I agree that there’s probably a little difference in battery aging between different speed chargers, however for overnight charging, where I really don’t care how long it takes to charge, I would still go with the lower power.
I would also have guessed the slow charging was better, but that is an uninformed opinion. I'd be quite interested in what people think.
An extra dilemma - often if I'm around the house or the office all day, I'll just plug in my phone every time I see it reach about 70%, and unplug when it reaches 80% (this is easier to spot if you have low-power mode on for an iPhone because the screen lights up at 80% when low-power mode is turned off). Is it worse to do that 2 or 3 times, compared to just charging to 100% and leaving it on the charger all day. Is being at 100% all day, but not using any (fractions of) charge cycles, worse than doing a few shallow discharges?
Of course, I don't think it really matters, but it's the kind of thing that I wonder and then do when I remember.
That said, my assumption is that most people want to charge overnight precisely because they do not want to have to remember to do a fast charge in the morning. Maybe that’s just me tho. Assuming you want to charge overnight, I totally agree that spending less time between 80% and 100% is good, and software that either prevents this or limits it is good. But whether you have such a device or not, for overnight charging a lower power charger seems prudent.