If you had it working before, you can probably re-create it using one of the various new bridging methods, but it may require some work.
One of the easiest, at least in terms of finding prebuilt pieces, might be to use MQTT, although you will need to set up an MQTT to KNX integration as well as a SmartThings to MQTT integration.
Here’s the thread on a community built project for that second piece if it’s of any interest:
Integration Solutions using MQTT
The same author also has a project called “edgebridge“ which lets you set up local API calls, but that would require you to do all the programming. It might be worth it, depending on your technical background.
You can ask further questions about either of those in their individual threads.
Longterm, the problem may be solved by Matter integration. Matter is a new industrywide home automation standard designed to allow for better inter-operability between different home automation systems. It’s being adopted by most of the big players, including SmartThings.
KNX hasn’t announced anything for its individual residential devices, but it is involved in a project for commercial lighting, which is intended to ultimately release a KNX-made device to bridge the older residential type lighting devices (which are not IP based) to a commercial KNX system which will be IP-based, and which will use thread, and will itself be integrated to matter.
If all of that happens and you have all of those individual pieces, you should then be able to add that to a smartthings account without needing any custom code or other pieces.
But that’s not here yet, there’s no announced timeline, and I don’t know what the financial cost would be of adding the additional bridge system. It may be that that’s only something that’s practical for commercial lighting projects. So I’m mostly just mentioning it because these days, the first question that comes up when looking at any possible integration, is “are they going to have support for Matter?” . And the answer in this case, appears to be that the company is looking into it, but maybe not in a way that would be practical for residential systems.
Anyway, all of that is to say that I think you could likely re-create what you had before using either MQTT or edgebridge, but I haven’t heard of anyone doing it yet, so it might be quite a bit of work.