I’m a South Africa married to a Norwegian. We lived in the UK for two years where I had a valid UK Residence card (residence card of a family member of a union citizen). We now live in Norway for which I have my Norwegian/EU residence card (residence card family member). Since my UK residence card has not expired, can I use it to travel back to the UK unaccompanied As before?
Your card is not expired, but it is no longer valid. From The Immigration (European Economic Area) Regulations 2016, as amended, section 18 (Issue of residence card):
(7) A residence card—
(c) is no longer valid if the holder ceases to have a right to reside under these Regulations;
Therefore, you may not use your UK residence card, and to travel to the UK without your spouse, you must get a visa.