There is a solution called Radiance Cascades [1] which doesn't require a denoiser for rendering real-time shadows for volumetric lights. Unfortunately the approach is relatively slow, so solutions based on denoising are still more efficient (though also expensive) in terms of the quality/performance tradeoff.

One issue with modern ReSTIR path tracing is that currently the algorithm relies on white (random) noise, which contains low-frequency (large-scale) noise, which produces blotchy boiling artifacts at low sample counts. Optimally an algorithm should use some form of spatio-temporal blue noise with exponential decay to only get evenly distributed high frequency samples. But that's still an open research problem.

1: https://youtube.com/watch?v=rG2aok2SdbU