Some community members have built their own integrations, typically using BroadLink, but sometimes using other bridges. People have mentioned a few of those below.
If you’re willing to use two bridges, there is a community-built project for integrating with an MQTT server on the new smartthings architecture. And it should be pretty easy to find a 433 MHz bridge that can work with MQTT. So that might be a possibility if there’s a particular 433 MHz bridge you want to use and you couldn’t find another integration for it.
There are other, albeit more expensive, home automation hubs like Homey which do include 433 MHz integration. However, smartthings has not shown any interest in adding more protocols. Their primary customer base is people who have Samsung smart appliances or televisions and don’t have a hub at all. So they seem most focused on supporting Wi-Fi and matter now. (They even dropped zwave from their newest hub, the Station.) so there haven’t been any official announcements, but I wouldn’t expect them to add 433 MHz support.
you can get cheaper/used smartthings devices on ebay. though keep in mind that there is usually no warranty. also, ebay is pretty strict with sellers and if they arent honest about what they are selling upfront, just open a case for a return so that the seller knows you mean business and have the backing of ebay if they dont refund or respond after a week. in all my years and purchases on ebay ive only had about a handful (5) such cases and most of the time the seller will refund upfront without sending the item back as it is going to cost them more with return shipping.