I don't mind 480p upscaled with a decent filter. Depending on the content, even 240p can work. What kills watchability for me is 480i. Maybe I just don't understand how things are suppposed to be done on modern setups, but it seems like the combination of deinterlacing plus upscaling is something that used to "just work" ca. 15 years ago and now it's almost impossible to get good results without either hooking up an old DVD player or getting a Ph.D. in ffmpeg.

So much this and it really sucks for older sub-720p content that isn't released on Blu-Ray because of the low resolution so for non-interlaced versions you have to resort to streaming services (no thanks) or the high seas. Even 720p series often never got Blu-Ray releases so you are stuck with both interlacing and a signficantly lower resolution than what should be available.

You have to understand the various TV types. OLED especially can look like crap if you don’t tune it right, because the pixels go on and off instantly, at least if what I heard was correct.

480i was built for the exact opposite, CRTs where pixels fade slowly.

OLED is irrelevant because the display isn't driven with interlaced data like a CRT TV might have been - instead the no matter the technology, all pixels are updated at the same time. The deinterlacing needs to happen before that and that was also needed for many displays back when DVDs were common.