Frozen vs fresh
Frozen food is more nutritious than fresh food. Frozen vegetables are picked at peak ripeness and flash frozen, whereas fresh food is picked while still ripening as they are expected to ripen in transit.
I haven’t seen any papers stating it impairs the fibre content but I can see where your train of thought is coming from.
It may depend on the method of freezing.Snap freezing with liquid nitrogen or dry ice leads to a better result and less cellular degradation than traditional freezing methods.
As someone who eats most food from frozen to eliminate wastage, I can confidently inform you that Bramley apples and rhubarb do well after traditional freezing.