Can't say about the safety as everyone over there is without mask. although dev deepawali is a surreal experience. if you are new I would suggest to stay around assi ghat aur dashashvamedh ghat for best experience and view of dev deepawali
I would suggest you to not attend this year's dev deepawali or even visit Varanasi until and unless the situation becomes a little stable as the real pleasure of exploring the city is when the lanes are crowded and as far as I know,people are being very careless these days without wearing mask.
Hey Sadhana , I was there few days back . I talked to a Sadhu who was doing Ganga Aarti he told me local administration didn't gave permission for dev deepawali this year. so there aren't any celebration this year . its not a good idea to go varanasi on dev Deepawali.