I would expect line 2x for 240p and 480i to 480p to work fine, so you should be able to use at least that; whereas Line 3x/4x/5x seems to be a crapshoot. Supposedly, 4K TVs in general are more flexible in what they support, and have a much better chance of supporting Line 3x/4x/5x than 1080p displays; and, even then, you may run into issues with the NES and SNES, which are known to have jittery sync signals that modern digital displays can’t handle. You probably won’t know what your TV supports until you get an OSSC and try it out.
Support may also depend on the model and/or region of TV you have. For example, TCL TVs sold in the US are reported to work fine with Line 3x/4x/5x, but the TCL TVs sold in Europe are entirely different. You may also find that some models from a given manufacturer will work fine, while others from that same manufacturer will not.
If you do decide to get an OSSC, be sure to share your TV’s model number and your findings in the display compatibility thread.