The main cause of the shrub's ailment is a disease called powdery mildew, and it is very common on Euonymus. Weather plays a large role in disease outbreaks and plant susceptibility, and the shrub's location next to a building wall, which impedes air circulation, may be exacerbating this outbreak. While powdery mildew is mainly a cosmetic problem, persistent or extensive outbreaks will cause enough leaf loss to weaken a plant.

If intervention is necessary, you can try using a fungicide according to its label directions. It will not cure existing infection so much as it helps prevent new infections on healthy leaves. The plant may continue to shed leaves which are already heavily infected; ideally, these should be collected and disposed of (do not compost).
https://extension.umd.edu/resource/powdery-mildew-disease-trees-and-shrubs

The bare-bottomed appearance to this shrub could be due to lower light from a top-heavy shape to the plant, where upper branches spread wider than lower branches and their shadowing of lower leaves causes them to shed (and not grow back). Euonymus responds well to harsh pruning, so you could opt to cut the plant back drastically next spring and let it regrow so you can gradually trim it into a tapered shape (narrowest at the top, widest at the base) so the lower regions remain leafy. If the plant is sited in lots of summer shade, though, this may have limited benefit since that can still hamper dense foliage retention. If deer visit the yard, they may be browsing on and removing some foliage, as they definitely enjoy eating Euonymus.
https://extension.umd.edu/resource/pruning-shrubs-and-hedges

We do not see clear signs of Euonymus Scale, but as a common insect targeting Euonymus, you can be on the lookout for it if the plant struggles going forward. Here is more info.: https://extension.umd.edu/resource/euonymus-scale-shrubs

Miri

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