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I don't know about higher level, more expensive stuff, but many popular microcontrollers (like the ESP8266 and ESP32) have SPI flash onboard, without any built-in wear leveling.

You are thinking Linux class devices, this is about MCUs with a few MiB of external flash

And there's no specification on how that wear-leveling works (or doesn't work).

That is not true. See https://github.com/littlefs-project/littlefs/blob/master/DES...

Sorry, as a commenter on this pointed out, the comment I was replying to was probably talking about eMMC. For which indeed no wear levelling is specified by the JEDEC standard - it is a standard that defines the communication protocol between a device and a host system. Newer JEDEC standards define the interface to access vendor specific "health reports", the actual method of wear levelling isn't regularly disclosed by vendors.

I think the GP was commenting on the lack of specification of eMMC wear leveling, not Littlefs.

Sandisk has multiple documents on this. "They" is a pretty big paintbrush.

You never know if a marginal brand has licensed the Sandisk patents or implemented them correctly.