All of the newer STM32s have ROM-bootloaders that support UART- or even USB-flash.

For SWD you can one of the ST-Link clones or the free open implementations of it.

Except when they don't. Last time I tried an ST part, I specifically bought one that advertised UART bootloader.

It didn't. Every piece of documentation I could find said that it did, but it would never respond. Forums were full of people complaining about this problem for years with no response from ST. No datasheet or marketing update, no errata. You just get a useless chip and it's your problem now. They also never responded to any of my direct emails or messages.

Every time I've tried an ST part, it's been hell and I eventually gave up and used an Atmel part instead. Every time.