This is a good reference, but IMO page tokens need to expire, even if you aren't storing any data for them. Expiring tokens make it possible to change pagination formats over time (you only need to wait the expiry time while supporting both formats), eg to improve efficiency.

Also I'll add that if you support a pagination API that accepts args (e.g. to filter on different dimensions) you should include a hash of the args in the opaque next page token, and fail the call if the args change. This prevents some nonsense scenarios where you started paging by scanning index A but the user switched filters such that you'd instead scan index B, but you don't have the key for that column.