If a combination is identified as particularly unusual, it will appear to enthusiasts so it's unlikely there any flows that are ticketable that haven't been used ever
UK stations with less than 100 entries and exits in 2021/22
1. Elton and Orston, Nottinghamshire (40 entries and exits)
2. Teesside Airport, Darlington (42)
3. Stanlow and Thornton, Cheshire (44)
4. Denton, Greater Manchester (50)
5. Kirton Lindsey, North Lincolnshire (68)
6. Sugar Loaf, Powys (76)
Clearly the enthusiasts do not seem to be travelling to these in such numbers as to make every station combination likely.
If we take Denton as an example, at 50 trips a year it would take over 50 years at the current rate to make every combination possible if there were no repeats, which seem unlikely and extremely complex to coordinate.