Charging has seen massive improvements in smartphones, at least on the Android side of things. Companies are pushing charging solutions that deliver 50W, 65W, and even 100W of power. These fast-charging technologies reduce the need for huge batteries, though I would argue they still don't excuse companies packing smaller batteries on new smartphones. The biggest problem with all of these competing technologies is that they're usually proprietary, so they require that you own the manufacturer's charging cable and brick. There is an open fast-charging standard for devices with USB Type-C ports called USB Power Delivery (USB-PD), but many times devices with proprietary charging technology are incompatible with USB-PD chargers. Google is pushing OEMs to change this.

USB Power Delivery can theoretically deliver up to 100W of power, but few smartphone manufacturers fully rely on USB-PD for fast charging and none of them approach 100W of power. The fastest I've seen is Samsung's 45W charger for the Galaxy Note 10 which uses PD 3.0 with Programmable Power Supply, or PPS. I can't speak for why OEMs other than Google and Samsung aren't embracing USB-PD, but Google has been working behind-the-scenes for a few years now to make sure that at the very least, devices with proprietary charging solutions aren't breaking compatibility with standard Type-C chargers. In fact, the Compatibility Definition Document for Android 7.0 Nougat, published in late 2016, contained wording that "strongly recommended" OEMs to "not support proprietary charging methods that...may result in interoperability issues with the chargers or devices that support the standard USB Power Delivery methods." Although Google wasn't enforcing any changes at the time, Google warned that "in future Android versions we might REQUIRE all type-C devices to support full interoperability with standard type-C chargers."

Section 7.7.1 "USB Peripheral Mode" from the Compatibility Definition Document for Android 10. The wording introduced with the CDD for Android 7.1 Nougat is still present.

Sometime in the past year, Google decided to make this "strong recommendation" into a requirement, at least for devices that ship with Google's apps and services. We obtained a copy of version 7.0 of GMS Requirements, published September 3rd, 2019. This document outlines the technical requirements that smartphone device makers must meet in order to preload Google Mobile Services (GMS), a suite of Google apps and services including the Play Store and Play Services. Nearly every Android smartphone or tablet sold internationally has met these requirements because having access to Google apps is critical for sales outside of China. Subsection 13.6 is titled "USB Type-C Compatibility" and it contains the following wording:

New DEVICES launching from 2019 onwards, with a USB Type-C port MUST ensure full interoperability with chargers that are compliant with the USB specifications and have the USB Type-C plug.

The wording in this statement is a bit ambiguous because "full interoperability" isn't made clear here. Judging by the phones that have already launched in 2019 like the OnePlus 7 Pro and OnePlus 7T, it's clear that Google isn't actually mandating that devices support the higher Power Rules like >27W or >45W via USB-PD support. This is evidenced by the fact that the OnePlus 7 Pro and OnePlus 7T support the "5V3A standard of Power Delivery" when connected to a PD-compatible charger. To better understand how USB-PD works, I recommend reading this excellent article from Android Authority.