The common use is estar, but if you're stating your status more formally, e.g. filling in a form or maybe an interview, the use of ser is not unusual. The reason to use mainly estar in with relationship statuses is because these convey some sort of "result":
Estoy soltera (because up to now I haven't married)
Estoy viuda (because my husband died)
Estoy casada (because I married someone)
They seem like results after having or not having done something.
I agree with Laura. SpanishDict has numerous examples of using *ser* with *soltero*, so maybe it is a regional difference.
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