Sometimes, I make a sentence whose subject can represent something of the past and something that still exists today. Which tense should I use for the helping verb? For instance: Any cellphone which (is/was) released between 2012 and the present year has this "mute" feature.
The time phrase in your sentence "between 2012 and the current year" clearly refers to a point in time in the past. So a past tense must be used.
Your sentence uses a passive construction which may be confusing you. Consider:
Apple releases/released the iphone-x between 2016 and the current year.
You should decide that the correct tense to use is "released".