A 9800 GT would work very poorly with OBS. It would technically "work" (in the sense that you wouldn't get the DirtectX 10 error) but it would perform so badly that you wouldn't be able to use OBS. That's not a guess, that's an observed fact, having personally seen several people try to do it. It's not worth it.
Support for older GPUs will never be added to the original OBS. Support for more GPUs was expanded in OBS Multiplatform, but you're still going to have a bad time of it. I'm sure some of the "bugginess" you were experiencing in Multiplatform was due to your old GPU.
Support for older GPUs will never be added to the original OBS. Support for more GPUs was expanded in OBS Multiplatform, but you're still going to have a bad time of it. I'm sure some of the "bugginess" you were experiencing in Multiplatform was due to your old GPU.