My in-laws and my parents are referred to by their first names by their children’s partners.
My nan (in her 90s now) insists on being ‘mom’ or ‘nan’ to everyone in the family, including her children or grandchildren’s partners. My DH still doesn’t refer to her directly by name when we see her as he finds it odd calling her nan (which I completely get, she’s not his nan!), so he just avoids using her name. She’s very old fashioned in that way though, only people of her generation are allowed to call her by her first name, everyone else is either mom/nan/mrs x. One of my cousins cousins (so no blood relation to her) had always called her nan as a child, as a teen he once turned to her and used her first name, everyone went silent and her face was like thunder.